Instructions

• Preheat the oven to 350°F (175°C). Grease five 8-inch round cake pans with cooking spray and dust lightly with flour, tapping out the excess. If you do not have five pans, plan to bake in batches.
• In a large mixing bowl, cream 1 cup softened butter and 2 cups sugar with an electric mixer until light and fluffy, about 2–3 minutes.
• Add 2 teaspoons vanilla extract to the butter mixture, then add 6 large egg whites one at a time, beating after each addition until fully incorporated and the batter is smooth.
• In a separate bowl, whisk together 3 cups all-purpose flour, 4 teaspoons baking powder, and 1 teaspoon salt.
• With the mixer on low, add the dry ingredients to the butter mixture in three additions, alternating with 1 cup milk (start and end with the dry ingredients). Scrape the bowl sides as needed and beat just until the batter is smooth.
• Divide the batter evenly among five bowls. Add one food coloring color to each bowl (red, orange, yellow, green, blue) and stir each bowl until the color is uniform.
• Pour each colored batter into a prepared 8-inch pan and smooth the tops.
• Bake the layers for about 15 minutes, or until a toothpick inserted into the center of each layer comes out clean. If baking in batches, allow pans to cool slightly before reusing.
• Let the cakes cool in their pans for 15 minutes. Run a knife around the pan edges to loosen, invert each layer onto a wire rack, remove the pan, and allow the layers to cool completely before frosting.
• Make the frosting: In a medium bowl, beat ½ cup softened butter with 4 cups powdered sugar until combined. Add 1 teaspoon vanilla and 2 tablespoons milk and beat until smooth. If the frosting is too thick, add up to 1 more tablespoon milk (use 2–3 tablespoons total). If it is too thin, add more powdered sugar until spreadable.
• Assemble the cake: place the blue layer on your serving plate (flat side down if you trimmed domes). Spread an even layer of frosting over the blue layer, then stack the green layer and frost, then yellow, then orange, then red, frosting between each layer. Use the remaining frosting to cover the top and sides of the stacked cake.
• Decorate the top with sprinkles of your choice. Allow the frosting to set slightly before slicing and serving.