This post may contain affiliate links. Please see my disclosure policy for more information.
This Rainbow Mousse Cake is seven gorgeous layers of cake and mousse in vibrant, fruity flavors. It’s a color and flavor explosion that is perfect for any party or celebration!
Happy blog birthday to ME!
This month marks 3 years of SugarHero-ing it up with you guys, making ridiculously tall cakes, using unhealthy amounts of butter, singlehandedly supporting the chocolate industry, and putting luster dust and gold leaf on absolutely anything and everything, darlings. It has been a blast and it just keeps getting better.
For the last two years, I’ve celebrated my blog’s birthday with an over-the-top rainbow-themed dessert (see these Rainbow Cake Push-Up Pops, and these Birthday Present Mini Cakes.) To me, nothing feels more celebratory than a big ole dessert packed with layers of bright colors. Pass the food coloring!
This year’s version is a seven-layer beauty alternating layers of moist butter cake and rich mousse. Each colorful layer is a different fruit flavor, so this cake is bursting with bright, sweet-tart flavor. I thought about calling it a “Taste the Rainbow Mousse Cake,” but it doesn’t actually contain Skittles (plus I’m scared of getting on the wrong side of the powerful Big Candy industry) so I kept things simple.
Here’s the actual breakdown of the cake: four layers of cake (lime, blueberry, raspberry, and lemon) sandwiching three layers of mousse (blackberry, cranberry, and passion fruit). The layers are flavored with a mix of extracts, zests, fruit purees, and powdered freeze-dried fruit. (Oh freeze-dried fruit, I wish I could quit you!) Although it could be made with just extracts, I love that each layer contains actual fruit products in the form of juice or puree or powder, so each has a different bold, vibrant realistic taste. You’ll notice that I used a lot of tart fruits, which went a long way toward keeping this cake from being too sweet, cloying, or overwhelming.
The cake is topped with a quick buttercream, lots of colorful sprinkles, and every rainbow candle and lollipop I could find. After all, you can’t spell “blog birthday” without “star-shaped rainbow lollipops,” right? (Oh hush.)
This cake is as fun to eat as it is to look at! It’s sweet, for sure, but also refreshing in the way that the best fruit desserts are. Jason basically deconstructed his slice, and mixed-and-matched the flavors, trying new combinations of fruits, just like you might do with a handful of Skittles. And of course, since it’s such a tall cake, it serves a ton of people, making this perfect for a big celebration.
It was a big culinary project, but I can’t think of a better way to celebrate three years of sharing big, beautiful desserts with you in this space! I have some huge plans for the next year, and I hope you’ll be along for the ride. Thank you for reading!
Rainbow Mousse Cake
For the Cake Layers:
- 14 oz granulated sugar 2 cups
- 16 oz cake flour 4 cups
- 1 tsp baking powder
- 1 tsp baking soda
- 1 tsp salt
- 4 eggs at room temperature
- 11 oz full-fat sour cream 1 1/3 cup, at room temperature
- 12 oz unsalted butter at room temperature
- 1 tsp lemon extract
- 1 lemon large
- 1 tsp lime extract
- 2 limes large
- 1 oz freeze-dried raspberries 1 cup
- 1 oz freeze-dried blueberries 3/4 cup
- Yellow gel food coloring I used Americolor Lemon Yellow
- Green gel food coloring I used Americolor Leaf Green
- Pink gel food coloring I used Americolor Deep Pink
- Purple gel food coloring I used Americolor Regal Purple
For the Passion Fruit Mousse:
For the Cranberry Mousse:
For the Blackberry Mousse:
- 9 oz blackberries fresh or defrosted from frozen
- 9 oz white chocolate chopped
- 1 1/4 cups heavy cream divided use
- Pinch salt
- 2 tsp unflavored powdered gelatin
- 2 tbsp cold water
- Blue gel food coloring I used Americolor Sky Blue
For the Vanilla Frosting:
- 12 oz powdered sugar 3 cups, sifted after measuring
- 4 oz unsalted butter 1 stick, at room temperature
- 2 tbsp milk plus more if necessary
- 1 tsp vanilla extract or other flavoring extracts
- 1/8 tsp salt
To Make the Cake Layers:
- Preheat the oven to 300 F. (That is not a typo—I use a low and slow bake time for these, to help prevent the cakes from doming.) Line four 9-inch cake pans with parchment rounds, and spray the parchment and pan sides with nonstick cooking spray.
- Combine the sugar, cake flour, baking powder, baking soda, and salt in the bowl of the stand mixer fitted with a paddle attachment. Mix on low speed for about a minute to combine the dry ingredients.
- Lightly whisk together the eggs and 1/3 cup of sour cream, and set aside for now.
- Add the remaining 1 cup sour cream and the softened butter to the large mixing bowl and mix on low speed until the dry ingredients are moistened. Turn the mixer to medium speed and mix for a minute and a half.
- Add the egg mixture in three batches, mixing for 20-30 seconds after each addition, until you have a smooth batter. Scrape the bottom and sides of the bowl with a rubber spatula, and give the batter a few more stirs to make sure everything is incorporated. Divide the batter into four equal portions—if you are using a kitchen scale, this will be about 15 oz of batter per bowl.
- To one portion of the batter, add the zest from one large lemon, 1 tsp lemon extract, and a few drops of yellow food coloring. Taste, and add more extract if desired, then scrape into one of the cake pans and smooth.
- To the second portion of the batter, add the zest from two large limes, 1 tsp lime extract, and a few drops of green food coloring, with an additional drop or two of yellow. Scrape into the second cake pan.
- Place the freeze-dried raspberries in a food processor and process until they're a fine powder. Add the raspberry powder and some pink food coloring to the third portion of batter, and gently stir until it's incorporated. Try not to over-mix, or your cakes will be tough. Pour into the third cake pan.
- Process the freeze-dried blueberries in the food processor, and add the blueberry powder and purple coloring to the final portion of batter. Pour into the fourth cake pan.
- Bake the cakes at 300 F for 20-25 minutes, rotating halfway throughout, until the tops spring back when lightly pressed and a toothpick inserted into the center comes out clean.
- Let the cakes cool on a wire rack for 10 minutes, then gently turn them out and let them cool completely. If you are making these in advance, they can stay in the freezer for up to several weeks.
- Prepare your pan. You need a 9-inch pan with tall sides, and the ability to remove the bottom. Either a cake pan with a removable bottom or a springform pan will work. Place a cake cardboard in the bottom of the pan. Tape two long strips of an acetate cake collar together, to make one 8-inch tall strip of acetate. Line the inner wall with your extra-tall acetate collar. Make sure the acetate is long enough so that it overlaps itself. I strongly recommend using acetate—it will provide you with a firm structure in which to build your cake, and clean edges along the sides. If you don't have acetate collars, you can try using a double layer of heavy-duty foil, 8 inches tall, to wrap around the inside of the cake pan. This should be stable enough, but it won't produce the clean edges that acetate does.
- Set your lemon cake layer in the bottom of the cake pan on the cardboard. Prepare your passion fruit mousse (instructions below), pour it on top of the lemon cake, and smooth it into an even layer. You'll notice that the mousse is somewhat loose and fluid, which is why it's so important to have a stable layer of acetate or heavy duty foil around the sides of the cake, to prevent the mousse from overflowing the sides.
- Chill the cake for 10 minutes, so the mousse just starts to set but is still tacky to the touch. Add the raspberry cake layer on top, and return to the refrigerator while you prepare the cranberry mousse layer (instructions below).
- When the cranberry mousse is done, pour it on top of the raspberry cake, and spread it into an even layer. Chill the cake again for 10 minutes, then add the blueberry cake on top. Refrigerate again while preparing the final mousse, the blackberry layer (instructions below.)
- When the blackberry layer is done, pour it on top of the blueberry cake and smooth it. Chill for 10 minutes, then top with the final lime cake layer. Refrigerate the cake for at least 6 hours (longer is okay!) to firm up all the mousse and make it a stable, solid cake.
- To serve, remove the cake from the cake pan, then carefully unwrap the acetate or foil collar from around the cake. Top with vanilla frosting (recipe below) or whipped cream, and lots of sprinkles and candles! This cake is easiest to cut when it's straight from the refrigerator, but I like it best when it's sat at room temperature for 20-30 minutes, so the cake is not completely cold.
To Make the Passion Fruit Mousse:
- Combine the white chocolate, passion fruit puree, 1/4 cup of cream, and salt in a large microwave-safe bowl. Microwave in 30-second increments, stirring every 30 seconds, until the chocolate has melted and the mixture is smooth. Let it cool to room temperature, stirring occasionally. It should not be warm to the touch at all.
- While you wait for the white chocolate mixture to cool, prepare the gelatin. Whisk together the gelatin and the cold water in a small bowl, and set it aside to let the gelatin absorb the water. When the chocolate is at room temperature, microwave the bowl of gelatin for 15 seconds, until it is melted. Whisk the melted gelatin and white chocolate together. Add several drops of orange food coloring, and stir them in. The color will get very diluted when you add the whipped cream, so go darker than your intended color.
- Whip the remaining 1 cup heavy cream to firm peaks. Fold half of the whipped cream into the white chocolate, and once it’s incorporated, gently fold in the remaining whipped cream. Try not to fold more than necessary, or the mousse will start to get grainy and break. Use immediately after it's made.
To Make the Cranberry Mousse:
- Follow the instructions for passion fruit mousse, but with a few small changes: combine the white chocolate, 1/2 cup cream, and salt and microwave together. Use red food coloring instead of orange, and add enough to make a very bright red color. Place the freeze-dried raspberries in a food processor and process until they're a fine powder. After you fold in the whipped cream, add the cranberry powder and stir it in gently.
To Make the Blackberry Mousse:
- First, blend the blackberries in a blender or food processor, then strain them to remove the seeds. Measure out 1/2 cup puree (save any additional puree for another use) and combine 1/2 cup puree, the white chocolate, and the salt in a microwave-safe bowl. Follow the instructions for the passion fruit mousse, but use blue food coloring instead of orange.
To Make the Vanilla Frosting:
- Place all of the ingredients in the bowl of a large stand mixer fitted with a paddle attachment. Mix on low speed until the sugar is moistened. Scrape down the sides of the bowl, then beat again on medium speed until light and fluffy, 2-3 minutes. You can add more milk or sugar, if desired, to get the consistency you want.