Preheat oven to 350°F and line a muffin tin with cupcake liners.
In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
In a large bowl, cream together butter and sugar until light and fluffy.
Add egg whites and vanilla extract. Beat until well combined.
Mix in sour cream, then add the dry ingredients and milk alternately until batter is smooth.
Gently fold in sprinkles.
Spoon batter into cupcake liners, filling 2/3 full.
Bake for 18-20 minutes or until a toothpick inserted comes out clean.
Cool completely before frosting.