Preheat oven to 350℉. Grease 9x13 pan.
In a bowl, whisk together cake flour, baking powder, baking soda, and salt. Set aside.
Lightly whisk the egg whites together in a bowl, set aside.
In a stand mixer, cream the butter and sugar for 2-3 minutes on medium-high speed.
Add the eggs to the mixture and beat on medium until incorporated.
Mix in vanilla and sour cream until creamy. Scrape down sides of the bowl as needed.
Slowly add the dry ingredients and alternate adding the buttermilk and milk in three additions with dry ingredients.
By hand, fold in the sprinkles until combined.
Pour batter into a greased 9x13 pan.
Bake for 35-40 minutes until a toothpick comes out clean. This cake comes out flat.
Allow the cake to cool before frosting.