In a small bowl, whisk together the egg whites, 1/4 cup of whole milk, and lemon zest. Set aside.

In a mixing bowl, beat the softened butter and granulated sugar until creamy, about 2-3 minutes. Add the cake flour, baking powder, and salt and mix until combined. Gradually add the egg white mixture, then beat in the remaining milk until the batter is smooth.

Prepare cupcake tins with paper liners. Fill liners about half full with batter. Bake in a preheated oven at 350°F (175°C) for 15 minutes. Remove from oven and let cool in the tins for 5 minutes before transferring to a wire rack to cool completely.

For the frosting, beat the softened butter for 3-5 minutes until pale in color. Add the powdered sugar, lemon zest, lemon juice (about 2 tablespoons from a fresh lemon), and heavy cream. Beat for another 3-5 minutes until light and fluffy.

Once the cupcakes are completely cooled, frost them using a piping bag or spatula. Enjoy!