Preheat the oven to 350 degrees F. Line a 12-cup muffin pan with paper liners. Whisk the flour, baking powder and salt in a bowl. Beat the butter and sugar in a stand mixer fitted with the paddle attachment on medium-high speed until fluffy, about 4 minutes. Beat in the eggs, one at a time, scraping down the bowl as needed. Beat in the vanilla. Reduce the mixer speed to medium low; beat in half of the flour mixture, then all of the milk, then the remaining flour mixture until just combined.
Divide the batter among the muffin cups, filling each three-quarters full. Bake until a toothpick inserted into the center of a cupcake comes out clean, 18 to 20 minutes, rotating the pan halfway through. Transfer the pan to a rack and let cool 5 minutes, then remove the cupcakes to the rack to cool completely. Top with Frosting.
Yield:about 2 cups
Whisk the egg whites, sugar and salt in a heatproof bowl set over a saucepan of simmering water (do not let the bowl touch the water) until the mixture is warm and the sugar dissolves. Remove the bowl from the pan; let cool slightly.
Beat the egg white mixture in a stand mixer fitted with the whisk attachment (or with a hand mixer) on medium-high speed until stiff peaks form, 12 to 15 minutes. Beat in the butter a few pieces at a time, then continue beating until smooth. (Don't worry if the mixture looks separated at first-it will come together as you keep beating.)
Make sure that your butter is nice and soft (with no chill) or the buttercream may curdled when mixing.