Preheat oven to 350F. Line mini cupcake pan with liners. Set aside.
In a large bowl, cream together the butter and sugar until light and fluffy, about 4 minutes.
Mix in vanilla extract (or vanilla bean paste) until combined.
Add 3 egg whites and beat on medium speed until thoroughly combined, about 30 seconds.
Add in remaining 2 egg whites and beat for another 30 seconds or until combined.
In a medium bowl, whisk together flour, baking powder, and salt.
Add half of the flour mixture to the wet mixture and mix until combined, scraping down sides of the bowl as necessary.
Add milk and mix until combined.
Add remaining flour mixture and mix until combined, scraping down sides of the bowl as necessary.
Fold in sprinkles carefully so the colors don't run.
Evenly divide the batter between 48 liners.
Bake for 12 to 14 minutes or an until an inserted toothpick comes out clean.
Cool cupcakes in pans for 7 to 8 minutes before removing from the pans and placing on a cooling rack. Let cool completely.