Rainbow Cake

Directions

1. 1
   Preheat oven to 375°F. Grease however many cake pans you have, and line the bottoms with parchment paper. (The recipe makes 7 layers of cake. I have 4 pans and I just reused them.)
2. 2
   In a large bowl, sift flour, baking powder, baking soda and salt. Mix well.
3. 3
   In the bowl of an electric mixer fitted with the paddle attachment, cream butter on medium-high speed, until light and fluffy. Gradually add sugar and continue to beat until well combined and fluffy. Reduce the mixer speed to medium and slowly add eggs until just blended.
4. 4
   With the mixer on low, slowly add half the flour mixture; mix until just blended. Add the milk and vanilla extract, mix until just blended. Slowly add remaining flour mixture, scraping down sides of the bowl with a spatula, as necessary, until just blended.
5. 5
   Divide batter evenly into 7 small bowls. Add 2 drops of food coloring paste to each bowl (1 color for each bowl) and mix thoroughly.
6. 6
   Pour into the prepared pans, smoothing the surface with a spatula. Bake 13 to 15 minutes or until a toothpick inserted into the cake comes out clean.
7. 7
   Place the cakes (in the pans) on a wire rack to cool. Then invert the cakes onto a greased rack. Cool completely before frosting.
8. 8
   To prepare the buttercream, put egg whites and sugar in a mixing bowl over a pot of simmering water. Use an electric hand mixer, beat constantly until the sugar has completely dissolved and the egg whites are hot. Remove from heat.
9. 9
   With a whisk attachment, continue whipping the mixture in a standing mixer until thick, glossy, and cool.
10. 10
    Switch over to paddle attachment. While mixing on medium speed, continuously add softened butter in chunks until incorporated, and mix until it has reached a silky smooth texture. If it curdles, keep mixing and it will become smooth again. Add vanilla and salt, mix well.
11. 11
    To assemble the cake, place one cake layer on your serving plate and spread with 1/4 cup of the buttercream. Repeat this process until all cake layers are placed. Smear buttercream on top and sides of the cake.
12. 12
    Garnish with color sprinkles.
13. 13
    Cover and refrigerate the cake. Leave out in room temperature for at least 1 hour prior to serving.