In a small bowl, mix egg whites, 1/4 cup whole milk, and lemon zest. Set aside.
In a mixing bowl, beat butter and sugar until creamy, about 2-3 minutes. Add in dry ingredients and mix until combined. Slowly add in egg white mixture. Beat in remaining milk.
Prepare cupcake tins with paper cupcake liners. Fill liners 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.
For the frosting, beat butter for 3-5 minutes until pale in color. Add powdered sugar, lemon zest, lemon juice (about 2 Tbsp from fresh lemon), and cream. Beat for 3-5 minutes until fluffy.
Frost cupcakes by filling a pastry bag with a tip and piping on the frosting. Enjoy!
Notes
These cupcakes are best enjoyed within a few days or can be frozen for longer storage.