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

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

Prepare cupcake tins with paper liners. Fill each liner 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: In a large bowl, beat softened butter for 3-5 minutes until pale and fluffy. Gradually add the powdered sugar, lemon zest, lemon juice, and heavy cream. Beat for another 3-5 minutes until the frosting is light and airy.

Once the cupcakes are completely cooled, frost them. You can use a piping bag with a tip for decorative frosting, or simply spread it with a knife. Enjoy!