Line a 12-count cupcake pan with paper liners. Set aside.

Add flour and baking powder to a small bowl and whisk until combined. Set aside.

Add the butter and sugar to a large mixing bowl using an electric mixer or bowl of a stand mixer fitted with a paddle attachment and mix with an until well combined.

Then, add the egg whites (ONLY THE WHITES) to the mixing bowl. Beat until creamy.

Add the vanilla to the milk and stir together.

Sprinkle in half of the dry ingredients and pour in half of the milk/vanilla to the mixing bowl and mix until well incorporated.

Add the remaining half of the dry ingredients and milk/vanilla and beat until smooth and fluffy.

Scrape the sides of the bowl with a rubber spatula and mix once more to ensure all ingredients are fully incorporated.

Pour the rainbow sprinkles into the batter and fold them in gently using a rubber spatula.

Pour the batter evenly into the prepared cupcake pan until each cup is approximately ¾ full.

Bake for 26-28 minutes, or until a toothpick inserted into the center of a cupcake comes out clean.

Remove the cupcakes from the oven and place the cupcakes on a wire cooling rack to cool completely.

To make the frosting, add the butter to a large mixing bowl using an electric mixer or bowl of a stand mixer fitted with a paddle attachment and beat until creamy.

Add the powdered sugar and begin to mix on LOW speed.

Pour in the milk, one tablespoon at a time, and mix on HIGH speed until smooth and fluffy.

Add the vanilla and beat for an additional minute or two.

To a piping bag fitted with a large star tip, add the frosting.

Pipe the frosting onto each cupcake in a swirl design.

Top with rainbow sprinkles if desired and serve.