Name: Rainbow Layer Cake
Yield: 1
Ingredients: 6 egg whites; 1 c. sugar; 1 c. unsalted butter; 3 tbsp. applesauce; 2 tsp. pure vanilla extract; 2 whole eggs; 2 1/4 c. cake flour; 2 tsp. baking powder; pinch of salt; 6 different hues of food coloring; 2 c. Buttercream Frosting (see Note); 2 c. rainbow sprinkles, nonpareils, and/or pearl dragées; 1 c. unsalted butter; 3 c. confectioners' sugar; 1 to 2 tablespoons heavy cream; 1 tbsp. pure vanilla extract; About 1/3 cup sprinkles

Step 1Make the Cake: Preheat oven to 350 degrees F. Line six 6-inch cake pans with parchment paper.

Step 2Beat egg whites with an electric mixer on medium-high speed until foamy, about 2 minutes. Increase mixer speed to high and continue beating until soft peaks form, about 3 minutes. Gradually add 1/2 cup of the sugar, continuing to beat until stiff peaks form, about 3 minutes more.

Step 3In another large mixing bowl, cream butter and the remaining 1/2 cup of sugar on medium speed until light and fluffy and increased in volume, about 5 minutes. Add applesauce, vanilla, and eggs and mix on medium speed for 1 minute more.

Step 4Sift together flour, baking powder, and salt. Add to butter mixture and mix on medium speed for 1 minute. Pour in 3/4 cup water and mix to combine. Fold in egg whites with a large spatula.

Step 5Divide batter evenly among 6 small bowls. Starting with the lightest color, tint batter with food coloring, whisking in a few drops at a time until the desired intensity is reached. Pour each of the colored batters into a different parchment-lined cake pan (each should be half full) and bake until a toothpick inserted in the center comes out clean, about 12 to 15 minutes.

Step 6Make the Buttercream Frosting: With an electric mixer on medium-high speed, beat butter for 1 minute, or until creamy and soft. Add confectioners' sugar, 1/2 cup at a time, beating to incorporate after each addition. Gradually beat in heavy cream until smooth.

Step 7Scrape down the sides of the bowl with a rubber spatula. Add vanilla—and sprinkles, if using—and beat for 1 minute more, or until soft and fluffy. Use immediately or refrigerate, covered, for up to 5 days.

Step 8Assembling the Cake: Stack layers according to the colors of the rainbow, spreading a thin layer buttercream between each. Using an offset spatula, frost the whole cake with a thin layer of buttercream.

Step 9Place cake on a freezer-safe plate. Freeze for 15 minutes, or until buttercream is firm. Remove from freezer and cover cake in sprinkles.