In a small bowl, mix egg whites, 0.25 cup whole milk, and lemon zest. Set aside.
In a mixing bowl, beat butter and sugar until creamy, about 2-3 minutes. Add dry ingredients and mix until combined. Slowly add the egg white mixture. Beat in the remaining milk.
Prepare cupcake tins with paper liners. Fill liners half full and bake in a 350°F oven for 15 minutes. Remove, cool in pan for 5 minutes, then transfer to wire rack to cool completely.
For the frosting, beat butter for 3-5 minutes until pale. Add powdered sugar, lemon zest, lemon juice (about 2 Tbsp), and heavy cream. Beat 3-5 minutes until fluffy.
Frost cupcakes using a pastry bag with a tip. Enjoy your lemon cupcakes!
Notes
These lemon cupcakes can be frozen before or after frosting for up to 3 months. Use the bounce-back test to check for doneness.