Preheat oven to 365°F. Line a 12-cup muffin tin with paper liners. Sift baking powder over flour in a bowl; whisk to combine. Measure milk using a liquid measuring cup and add the vanilla.

Using an electric mixer with paddle attachment, cream the butter until light in color and texture, 3–4 minutes, scraping down the bowl and beater once or twice. With the mixer running, sprinkle in the salt, then add the sugar in a slow stream, continuing to mix until light and fluffy, about 3 minutes more. Add the egg whites one at a time, mixing well after each.

Add the flour to the butter in three additions, alternating with the milk/vanilla. Mix briefly on low speed after each addition. Divide batter into the cupcake liners and bake for 14–16 minutes, or until the center springs back when you touch it lightly with your fingertip. Let cool for 5 minutes in the pan, then remove the cupcakes to a wire rack to cool completely before frosting.

Follow the recipe using these substitutions: milk ¾ cup, sifted all-purpose flour 1¾ cups (215 grams), sugar ¾ cup, baking powder 2 teaspoons, and bake at 350°F for 18–20 minutes.