This cheerful Grapefruit Layer Cake is sure to brighten your day! With moist grapefruit cake, a grapefruit curd filling, and grapefruit buttercream on the outside, it's wonderfully refreshing and such a nice change from the usual lemon or orange cakes. If you are using large, juicy grapefruit, you can probably get all the juice you need in this recipe from 2 fruits, but it's better to have 3 grapefruit available, just in case.
2tspunflavored gelatin powder(optional, see note below)
4ozunsalted butterat room temperature
10 1/2ozgranulated sugar(1 1/2 cups)
1cupfresh grapefruit juice
2tbspfresh lemon juice
Pink and orange food coloring
For the Grapefruit Buttercream:
8ozunsalted butterat room temperature
24ozpowdered sugar(6 cups)
1/4cupfresh grapefruit juice
Pink and orange food coloring
1 1/2lbsfondant(I like homemade marshmallow fondant)
Yellow, orange, ivory, and pink food coloring (I like Americolor gel brand)
To Make the Grapefruit Cake:
Preheat the oven to 350 F and line four 8" pans with parchment paper. (If you have 8" pans with tall 3-inch sides, you can bake this in 2 pans and adjust the baking time, then split the layers after baking to get 4 layers.) Place the sugar and grapefruit zest in the bowl of a large stand mixer. Rub them together with your fingers until the sugar is moist and very fragrant. Add the room temperature butter and beat them together with a paddle attachment until light and fluffy, about 3-5 minutes.
Scrape down the sides and bottom of the bowl, then with the mixer running, add the eggs one at a time, beating well after each addition. Add the grapefruit juice and mix it in.
In a separate bowl sift together the cake flour, baking powder, and salt. Add a third of the dry ingredients, then when they're mostly mixed in, add half the buttermilk and mix it in. Continue to alternate adding wet and dry ingredients, ending with the drys. Stop the mixer when most of the flour streaks have disappeared, and finish mixing by hand, being sure to scrape the bottom and sides of the bowl well.
Divide the batter between the four pans. If you have a kitchen scale, each pan should get 12 1/2 oz of batter. Bake the cakes for 25 minutes, rotating halfway through, until the tops spring back when lightly pressed and a toothpick inserted into the center comes out clean. Cool the cakes on a wire rack for 20 minutes before turning them out and letting them cool completely.
To Make the Grapefruit Curd:
Make the curd at least 4 hours in advance. (The curd can be made up to a week in advance and kept in the refrigerator until ready to use.) In a small bowl, combine the gelatin and cold water and whisk together. Set aside to let the gelatin absorb the water while you prepare the rest of the recipe.
Fill the bottom of a double boiler (or a regular saucepan) with about an inch of water and bring it to a simmer on the stovetop.
In a large mixing bowl, cream together the butter and the sugar with a handheld or stand mixer fitted with a whisk attachment until the mixture is light and fluffy. Slowly add the eggs and yolks, and beat for 1 minute more. Add the grapefruit and lemon juices and mix—at this point the mixture will look curdled.
Pour the liquid into the top of a double boiler or a bowl that fits snugly over your saucepan. (I actually like to use the metal mixing bowl that goes on my stand mixer so I’m not dirtying too many dishes.) Cook the mixture over the simmering water, whisking frequently, until it thickens enough to coat the back of a spoon. If you want to use a candy thermometer, cook the curd until it reaches 175 degrees F. This process might take 10-20 minutes, depending on the equipment you're using.
Once the curd thickens, remove the pan from the heat and add the gelatin. Whisk until the gelatin liquefies and everything is well-mixed. Add a few drops of pink food coloring and one drop of orange, to make a nice grapefruit color. Pour the curd through a fine mesh strainer into a bowl to remove any bits of cooked egg. Press a layer of cling wrap directly on top of the curd, and refrigerate it until it is chilled and thick.
To Make the Grapefruit Buttercream:
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 juice or sugar, if desired, to get the consistency you want. Once the buttercream is light and fluffy, add a few drops of pink food coloring and one drop of orange, to make a nice grapefruit color.
The buttercream can be made up to a week ahead of time and stored in an airtight container, with cling wrap pressed on top, for up to a week. Let it come to room temperature and re-whip to get it fluffy again before using it.
If any of your cakes have a strongly domed top, trip the tops so they're level. Place a cake on an 8" cardboard cake round. Fit a piping bag with a large round tip, and fill the bag with grapefruit buttercream. Pipe a thick line of buttercream all along the edge of the cake layer to form a "dam" and prevent the grapefruit curd from leaking out.
Whisk the curd gently to loosen it and break up any lumps, then spread a third of the curd on the cake, filling in the space in the buttercream circle.
Place the next cake layer on top of the first, and repeat the process of the buttercream dam and the curd filling. Continue until you've used all of the curd and cake layers. Spread a very thin layer of buttercream all along the outside of the cake, to seal in any crumbs. Refrigerate the cake for about an hour, or until the buttercream is firm, so that your cake is stable and can withstand the final decorating.
Spread a thicker layer of buttercream all along the top and sides of the cake. It will be covered by fondant, so it doesn't have to be perfectly smooth, but a smoother surface will help the fondant look better. A cake turntable and a metal offset spatula help make the decorating process easier! Refrigerate the cake again for another 1-2 hours, until the buttercream is very firm.
To cover the cake with fondant, combine 2 tbsp light corn syrup and a teaspoon of water in a small bowl. Brush this mixture over your cake in a very thin layer—this will help the fondant stick to your buttercream, which has developed a crust and is otherwise not very sticky.
Dust your work surface with powdered sugar, and roll out the fondant until it is a round a little less than 1/4-inch thick. Drape it over the cake and smooth it evenly against the top and sides—if you are unfamiliar with covering a cake with fondant, there are many excellent video tutorials to demonstrate this step. Trim the excess fondant from the bottom. If you're not going to decorate it immediately, wrap it in plastic wrap (to prevent condensation) and refrigerate it.
To decorate the cake like a grapefruit, start by adding some dimples to the sides of the cake. I used a new, clean plastic hairbrush with the small balls on the end of the bristles, but you can use a toothpick, skewer, or any other tool that can give the fondant a stippled effect.
In separate small bowls, combine a spoonful of water with a drop or two of each gel food coloring: yellow, orange, pink, and ivory. Use a large flat brush to paint a wash of yellow on the sides of the cake. If the color is too bright, add more water to your bowl, and if it's too light, add a bit more food coloring. After the sides are yellow, go back and add layers of orange here and there, blending well to make the colors mix. Add a bit of ivory toward the bottom to give the grapefruit sides more depth. Finally, paint a small margin of yellow-orange color around the top of the cake to represent the rind. It should only be about 1/4-inch thick.
If it helps, you can sketch out the sections on top of the cake before beginning by lightly dragging a toothpick across the top of the fondant. Leave a circle in the center unpainted, and try to leave a thin margin of white in between each section, and a thicker white margin around the edges. Paint each section pink, then go back and blend in orange and yellow colors. Finally, use a bit of undiluted pink food coloring to add subtle streaks to the grapefruit sections—this gives them a more realistic texture. Let the food coloring dry (for at least 30 minutes) before cutting and serving the cake.
When I make lemon curd to use as a cake layer, I like to add a little gelatin to stabilize it and produce clean, neat cake slices that can hold their shape when cut, and withstand sitting at room temperature. If you don't have gelatin, or don't eat it for some reason, the gelatin + water step can be omitted from the curd, and you'll just have a slightly looser filling as a result.