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

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

Prepare cupcake tins with paper liners. Fill each liner about two-thirds full with batter. Bake in a preheated oven at 350°F (175°C) for 15 minutes, or until a toothpick inserted into the center comes out clean. Let the cupcakes cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.

To make the frosting, beat the softened butter in a large bowl 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 smooth.

Once the cupcakes are completely cool, frost them using a piping bag and your desired tip, or spread with a spatula. Enjoy your delicious homemade lemon cupcakes!