Step 1: The "Sandy" Mix
Preheat your oven to 350°F (175°C). Grease three 6-inch pans (perfect for the tall, cute aesthetic) and line with parchment. In a stand mixer, combine cake flour, sugar, baking powder (2 ½ tsp), and salt (½ tsp). Add the butter chunks and mix on low until it resembles coarse sand.Step 2: The Whitening Phase
In a jug, whisk the egg whites, milk, sour cream, and clear vanilla. Pour half of this liquid into the sandy flour mixture. Beat on medium-high for exactly 2 minutes to build structure.Step 3: The Confetti Fold
Lower the speed and add the remaining liquid. Mix just until combined. Remove the bowl from the mixer. Gently fold in the rainbow sprinkles by hand using a silicone spatula.Step 4: The Even Bake
Divide batter evenly between pans. Bake for 30-35 minutes. The cakes are done when they are pale (not browned) and spring back when touched.Step 5: The Crumb Coat & Chill
Cool cakes completely. Stack them with layers of buttercream. Apply a thin "crumb coat" of frosting to trap any loose crumbs. Chill for 20 minutes.