Preheat oven to 350°F. Prepare cupcake tins with paper liners.

In a mixing bowl, beat butter and sugar until creamy. Add egg whites and lemon zest, mix well.

Gradually add dry ingredients and milk, mixing after each addition until smooth.

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

Let cupcakes cool completely before frosting.

Prepare lemon frosting. Pipe onto cooled cupcakes.