Instructions

• Preheat oven to 350°F. Spray two 9-inch round cake pans with nonstick cooking spray. Line the bottoms with parchment or waxed paper rounds, spray the paper, then dust the pans with cake flour; invert each pan and tap to remove excess flour. Set pans aside.
• In a medium bowl, whisk together 1 cup whole milk, 6 large egg whites, and 2 teaspoons vanilla extract until well blended. Set this milk–egg mixture aside.
• In the bowl of a stand mixer fitted with the paddle/beater blade, combine 2 1/4 cups cake flour, 1 3/4 cups granulated sugar, 4 teaspoons baking powder, and 1 teaspoon salt. Mix on low just to combine.
• Add 3/4 cup unsalted butter (room temperature) to the dry ingredients and beat on low speed until the mixture resembles moist crumbs.
• Measure and reserve 1/2 cup of the milk–egg mixture. Add the remaining milk–egg mixture to the mixer (the portion you did not reserve) and beat at medium speed for 1 1/2 minutes.
• Add the reserved 1/2 cup of milk–egg mixture and beat 30 seconds more. Stop the mixer, scrape down the sides and bottom of the bowl, then return to medium speed and beat an additional 20 seconds.
• Fold the rainbow jimmies (sprinkles) from the ingredient list into the batter by hand until evenly distributed.
• Divide the batter evenly between the two prepared pans and smooth the tops with a rubber spatula. Place the pans on the middle oven rack and bake 23–25 minutes, or until a toothpick inserted in the center comes out clean.
• Remove the pans from the oven and let the cakes rest in the pans for 3–5 minutes. Run a knife around the edges to loosen, invert the cakes onto wire cooling racks, remove the parchment rounds, and let the cakes cool completely.
• While the cakes cool, make the buttercream: in the stand mixer bowl, beat 1 1/2 cups unsalted butter (room temperature) on medium speed with the paddle/beater blade until smooth, about 3 minutes.
• With the mixer on low, add 5 cups sifted confectioners’ sugar to the butter 1/2 cup at a time, mixing until incorporated after each addition.
• Add 1 tablespoon vanilla extract and 1/4 teaspoon salt; mix to combine. Add 3–4 tablespoons milk or heavy cream, 1 tablespoon at a time, until the frosting reaches your preferred spreading consistency. If you want a thicker frosting, add more confectioners’ sugar 1/4 cup at a time; for thinner frosting, add more milk/cream 1 tablespoon at a time.
• If needed, use a serrated knife to level the tops of the cooled cakes by trimming a very thin layer from each to create even surfaces. Save the trimmings if desired.
• Place one cake layer (cut-side up if you trimmed, or top-side up if not) on a cake stand or serving plate. Spread an even layer of vanilla buttercream over the top. Sprinkle with the decorating sprinkles from the ingredient list, if desired.
• Place the second cake layer on top, cut-side down (so the tops are aligned and you have a smooth surface to frost). Frost the top and sides of the assembled cake with the remaining buttercream using an offset spatula or knife. Decorate the top and sides with additional sprinkles as desired.
• Slice and serve.
• Make-ahead and storage: cake layers can be baked, cooled, and wrapped tightly in plastic wrap the day before and stored at room temperature. Buttercream can be made the day before, stored in an airtight container in the refrigerator, brought to room temperature and re-whipped before using. Once assembled, keep the cake covered at room temperature for up to 2 days; it is best the day it is made.