In a small bowl, whisk together egg whites, 1/4 cup of the 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 kosher salt and mix until combined. Gradually add the egg white mixture, beating until just incorporated. Beat in the remaining milk until the batter is smooth.

Line cupcake tins with paper liners. Fill each liner about half full with batter. Bake in a preheated 350°F (175°C) oven for 15 minutes. Let cool in the pans for 5 minutes before transferring to a wire rack to cool completely.

For the frosting: In a clean mixing bowl, beat the 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 using a piping bag with your favorite tip, or spread with a spatula. Enjoy your delicious homemade lemon cupcakes!