Ingredients

Vanilla Cake
Double the recipe below to make six layers (as pictured).

4 large eggs
3 1/2 cups cake flour
4 teaspoons baking powder
1/2 teaspoon salt
1 cup unsalted butter at room temperature
2 cups granulated sugar
2 teaspoons pure vanilla extract
1 cup milk

Vanilla Buttercream Frosting

12 oz. unsalted softened butter
7 1/2 cup powdered sugar
1 1/2 teaspoon kosher salt
3 tablespoons plus 3 teaspoons whole milk
3 teaspoons pure vanilla extract
3 teaspoons fresh lemon juice

Instructions

1. Preheat oven to 350 degrees F. Line bottom of the pans with parchment paper. Separate egg whites in a small bowl and yolks in another. Set aside.
2. In a separate bowl, roughly mix flour, baking powder and salt. Beat butter until soft using a stand mixer. Add 1 1/2 cups sugar and mix until fluffy. Add egg yolks one at a time, and continue to beat. Add vanilla into mixture. Scrape the sides of the bowl to thoroughly combine. Bring mixer to low speed, and slowly add flour mixture and milk.
3. In a separate bowl for the stand mixer, whisk egg whites until foamy. Add 1/2 cup sugar until mixture forms stiff peaks. Gently fold in egg white mixture into the batter. Be careful to not over mix. Fill cake pans evenly. Bake on center rack for about 25 minutes or until fully baked. Allow cakes to completely cool.
4. To make the buttercream frosting, add 1/2 the powdered sugar to the bowl using a stand mixer with a paddle attachment. Add in butter, kosher salt, 3 tablespoons whole milk, vanilla and lemon juice. Beat until moist. Scrape the sides of the bowl to thoroughly combine all ingredients. Add the rest of the powdered sugar and milk, mixing thoroughly.
5. Separate frosting into six bowls, leaving one with a little extra frosting for the top layer. Mix in desired food coloring to each bowl.
6. If you are not using the frosting immediately, tightly wrap and store at room temperature for up to 2 days or refrigerate for one week. If chilled, let buttercream come to room temperature and stir well to release any air bubbles before using.
7. To assemble the cake, remove any excess cake on top of each round for a flat, even surface. For a six-layered cake, use a cake leveler or long serrated knife to evenly slice cakes in half.
8. Cover first layer with red frosting, allowing some to drip over edges just slightly. Repeat steps for each layer with orange, yellow, green and blue frosting. Cover the top layer with purple frosting, and spread until top is smooth. Transfer to a cake stand.