Preheat oven to 350°F. Line a 12-count cupcake tin with cupcake liners.
Whisk together the flour, baking powder, and salt. Set aside.
1¼ cups all-purpose flour, 2 teaspoons baking powder, ½ teaspoon kosher salt
In a large bowl, using a hand mixer, beat the butter and sugar together until light and fluffy.
6 tablespoons unsalted butter, ¾ cup granulated sugar
Stir in buttermilk and vanilla extract.
6 tablespoons buttermilk, 1 teaspoon pure vanilla extract
Add egg whites and mix until well combined.
3 egg whites
Add half of the dry ingredients to the batter and mix until well combined. Mix in the milk. Add remaining dry ingredients and mix until well combined.
6 tablespoons milk
Fill the cupcake liners about halfway. Bake for 15 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool completely on a cooling rack.