Preheat the oven to 350F. Line a muffin tin with cupcake liners and set it aside.
In a medium bowl, use a hand mixer to whisk the egg whites until soft peaks form, about 2-3 minutes. You want them to be very fluffy, with no liquid remaining in the bowl. Set this aside.
In a separate bowl, whisk together the flour, baking powder and salt. Set this aside.
Using either a hand mixer or a stand mixer with the paddle attachment, beat the butter on high speed until it is smooth and creamy, about a minute or so. Then add the sugar and beat together for 3-4 minutes. Scrape down the sides of the bowl as needed. Add the egg yolks and vanilla and beat until combined.
Turn the mixer to low speed and alternate adding the dry ingredients in 3 batches and the milk in 2. Start and end with the dry. Mix each until just incorporated.
Using a rubber spatula, gently fold in the egg whites. The batter should be slightly thick, smooth and velvety.
Scoop the batter into the cupcake liners, filling each about halfway. Bake for 17-21 minutes, until a toothpick inserted into the centers come out clean. Let them cool in the pan for 5 minutes, then remove them to a wire rack to cool completely.
Once they are completely cool, frost them with your favorite frosting recipe. Store covered for 3-4 days.
