Prepare each 6" cake pan by thoroughly coating with cooking spray. Then, line each bottom of pan with parchment paper and then give a quick spray over paper.
Preheat oven to 350 degrees F.
Start by creaming softened butter and sugar together with a hand mixer on medium/high speed. Blend until light and fluffy.
Next, add in vanilla extract and egg whites. Blend well between each egg added.
In another bowl, whisk dry ingredients together (flour, baking soda, baking powder, and salt).
To the bowl with wet ingredients, alternate adding in dry ingredients and buttermilk. Blend on high speed until everything is completely combined.
Stir in sprinkles by hand.
Pour batter evenly between three 6" round cake pans.
Bake for about 30-35 minutes or until toothpick inserted in middle of cake comes out clean.
Allow cakes to cool on cooling rack for about 20 minutes before trying to remove from pans.
Remove parchment paper from bottom of cakes. Allow cakes to cool completely before attempting to frost.