In a small bowl, mix egg whites, 1/4 cup whole milk, and lemon zest. Set aside.
In a mixing bowl, beat softened butter (for cake) and granulated sugar until creamy, about 2-3 minutes. Add in dry ingredients (cake flour, baking powder, salt) and mix until just combined. Slowly add in the egg white mixture from step 1. Beat in the remaining 1/4 cup of whole milk.
Prepare cupcake tins with paper cupcake liners. Fill liners about half full and bake in a 350 degree oven for 15 minutes. Remove and cool in pan for 5 minutes then transfer to a wire rack to cool completely.
Frosting Preparation
For the lemon buttercream frosting, beat softened butter (for frosting) for 3-5 minutes until pale in color. Add powdered sugar, lemon zest, lemon juice, and heavy cream. Beat for 3-5 minutes until light and fluffy.
Frost the cooled cupcakes by filling a pastry bag with tip and pipe on the lemon buttercream. Enjoy.
Notes
These cupcakes are best stored in an airtight container at room temperature for up to 3 days, or they can be frozen.