Preheat the oven to 350 degrees F. Line cupcake tins with paper liners.

In a mixing bowl, beat butter and sugar until creamy. Add egg whites one at a time, then add lemon zest.

In a separate bowl, whisk together flour, baking powder, and salt. Gradually add dry ingredients to wet mixture, alternating with milk, until smooth.

Fill cupcake liners half full with batter. Bake for 15 minutes or until a toothpick inserted in the center comes out clean.

Let cupcakes cool completely before frosting.

Prepare lemon buttercream by beating softened butter, powdered sugar, lemon zest, and lemon juice until fluffy.

Frost cooled cupcakes with lemon buttercream using a piping bag.