8ozchocolate candy coating or semi-sweet chocolate
Oreos or other cookies
Gold-wrapped candies like Ferrero RochersRolos, and chocolate coins
Assorted gold pearls or other gold sprinkles
To Make the Lemon Cake:
Preheat the oven to 350 F. Line four 8-inch cake pans with parchment rounds, and spray the parchment and pan sides with nonstick cooking spray.
Combine the sugar and lemon zest in the bowl of a stand mixer, and rub them together between your fingers until the sugar is moist and fragrant with lemon. Add the cake flour, baking powder, salt, and lemon gelatin powder to the bowl, and fit it with a paddle attachment. Mix the ingrdients on low speed for about a minute to combine the dry ingredients.
Lightly whisk together the egg yolks and about a quarter of the milk (it doesn’t have to be a precise measurement) and the vanilla and lemon extracts, and set aside for now.
Add the remaining milk 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 yolk 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 evenly. If you want to be precise, each pan should get about 13.5 oz batter.
Bake the cakes for 25-27 minutes, rotating them from top to bottom and back to front halfway through. Bake until the sides just start pulling away from the pan, 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. I find it’s easiest to cut and stack cakes that have been chilled, so if you have time, wrap them with plastic wrap and put them in the freezer for at least 30 minutes, until they are firm but not frozen all the way through. If you are making these in advance, they can stay in the freezer for up to several weeks. Let them partially defrost, until they are firm but not rock-hard, before assembling.
To Make the Lemon Buttercream:
Place the room temperature egg whites in the very clean bowl of a large stand mixer fitted with the whisk attachment. Place the water, lemon juice, and sugar in a medium saucepan over medium-high heat and stir until the sugar dissolves. Once the sugar is dissolved and the mixture has come to a boil, insert a candy thermometer and cook until it reaches 245 degrees F.
While you are waiting for the sugar syrup to reach the proper temperature, begin beating the egg whites. (I usually start beating them when the temperature is around 225 degrees.) Turn the mixer to medium speed and beat until the whites look foamy. Stop the mixer briefly and add the cream of tartar. Turn the mixer back on and begin to beat on medium-high speed until the whites are stiff but not crumbly. You can turn the mixer off if the egg whites are finished before the syrup is ready.
When the whites are stiff and the sugar syrup is 245 degrees, turn the mixer back on to medium-high and pour the syrup into a cup or pitcher with a spout. Slowly and carefully pour the hot syrup down the sides of the bowl into the egg whites while the mixer is running. The whites will rise in volume. After all of the syrup is added, let the mixer run on medium speed until the sides of the bowl feels barely warm, which may take anywhere from 10-20 minutes. The whites should be very glossy, white, and shiny, with stiff peaks.
Once the egg white mixture has cooled to room temperature, gradually add the butter bit by bit until it is all incorporated. Scrape down the sides of the bowl with a spatula. The egg white mixture will deflate and might look soupy. If it looks soupy or broken, continue to beat the mixer on low or medium speed until it comes back together. (This is why it’s important to use butter that’s soft but cool: butter that is too warm will just liquefy and make soupy buttercream.) If it resists coming together, refrigerate the bowl briefly, for about 10 minutes, then continue beating until it smooths out.
Once the frosting has come together, add the salt and lemon extract to taste. The buttercream can be made in advance and kept at room temperature if you’re going to use it the same day, or refrigerated. If it’s been chilled, let it sit at room temperature until it softens, then re-whip it to get the fluffy texture back before you use it.
Divide the frosting into 7 portions. Color each portion a different color using gel food coloring. (I used red, orange, yellow, green, light blue, dark blue, and purple, and those are the colors I'll reference in these instructions. You can use different colors if you wish.) Place each color in a disposable pastry bag, and snip off the tip.
Place the chilled cakes on cardboard cake rounds. If any of the cakes have a rounded top, gently trim off of the top so that they're flat. Use a sharp serrated knife to slice each cake in half. Place the bottom layer of your cake on a cake turntable.
Place about 3/4 cup of red frosting in the center of the cake, and use a metal offset spatula to spread it into an even layer out to the edge of the cake. It's fine if some frosting extends beyond the edge. Place a second cake round on top, and repeat the process with orange buttercream. Continue layering the cakes and frosting, working through your rainbow colors, until you have an 8-layer cake, finishing with purple on top. Spread a very thin layer of purple frosting on the top, and use your spatula to smooth the sides of the cake, spreading the frosting that has extended over the edges around the cake. Fill in any bald spots with a bit more frosting—the goal is just to have a very thin layer of frosting covering the entire cake, to seal in any crumbs. Refrigerate the cake for 30-60 minutes, to firm it up.
Once the cake has chilled, take your piping bags of frosting colors and thick pipe stripes around the side of cake, positioning them so they correspond to the colors on the inside. Finish with purple on the top. Take a metal offset spatula and smooth out the top of the cake, then clean it off and hold it gently against the sides of the cake. Turn the turntable while you hold the spatula against the side. Make a few rotations, until the frosting smooths into an even layer and the colors blend together a little bit. Chill the cake again for about an hour before decorating. You want the frosting to be set before you add the gold leaf.
To add the gold leaf to the top of the cake, you'll want a razor or similarly clean, pointy object, and a big fluffy brush—you can use either a makeup brush or a paint brush, but make sure that it's new and doesn't have any makeup or paint on it! Some gold leaf is sold on transfer paper, and if that's the case, it's easy to use—just separate the paper from the rest of the pack, press the gold side on top of the cake, then peel the transfer paper off. Use a big fluffy brush to press the leaf onto the cake. Repeat until the entire top of the cake is covered with gold leaf. Extend it a little bit down the sides of the cake as well.
If your gold leaf is sold as loose leaves, you'll need to be more careful. Hold the packet of gold leaves close to the top of the cake, and use a razor, knife, or other precise object to slide the leaf from the paper onto the cake. Don't use your hands, and don't do this in a windy area—gold leaf is very delicate and easily crumples, blows away, and sticks to human skin. It's okay if it wrinkles, just use the big brush to press it onto the cake—some texture in the gold actually looks good!
If you want to decorate with chocolate shards, melt the chocolate candy coating or semi-sweet chocolate in the microwave. Cover a baking sheet with parchment or waxed paper, and pour the melted chocolate onto the paper. Spread it into a thin, even layer, and refrigerate for 15-20 minutes, until set. Cut it into large shards.
Use edible gold spray or edible gold luster dust to color the chocolate shards gold. You can also spray or paint assorted cookies gold—I used Oreos and Pizelles.
Finally, it's time to decorate! Stick the chocolate shards into the top of the cake. Add various gold cookies, candies, and sprinkles around it, so that it looks like it's bursting with golden goodies. For the best taste and texture, serve this cake at room temperature.