Preheat oven to 350 degrees F. Line a 12-cup standard muffin tin with paper liners; set aside.

In a large bowl, combine flour, baking powder and salt.

In the bowl of an electric mixer fitted with the paddle attachment, beat butter and sugar on medium speed until light and fluffy, about 1 minute. Beat in egg whites until well combined. Beat in vanilla until well combined. With the mixer on low speed, add dry ingredients and milk alternately in 3 parts, beginning and ending with the dry ingredients, beating just until incorporated. Add sprinkles and stir using a rubber spatula just until moist.

Scoop the batter evenly into the muffin tray. Place into oven and bake for 18-20 minutes, or until a tester inserted in the center comes out clean. Remove from oven and cool completely on a wire rack before frosting.

In the bowl of an electric mixer fitted with the paddle attachment, beat butter on medium speed until smooth, about 3 minutes. With the mixer on low speed, gradually add powdered sugar, beating just until incorporated. Add vanilla, salt and milk. Increase speed to medium high and beat until frosting is light and fully, about 1-2 minutes.

Use a small offset spatula to spread the cupcakes with frosting, garnished with sprinkles, if desired.