In a small bowl, mix egg whites, 0.25 cup whole milk and lemon zest. Set aside.
In mixing bowl, beat butter and sugar until creamy, about 2-3 minutes. Add dry ingredients and mix until combined. Slowly add in egg white mixture. Beat in remaining milk.
Prepare cupcake tins with paper liners. Fill liners half full and bake in a 350°F oven for 15 minutes. Remove and cool in pan 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 for 3-5 minutes until fluffy.
Frost cupcakes using a pastry bag and tip. Enjoy!
Notes
These cupcakes are perfect for spring and summer gatherings. Store in an airtight container for up to 3 days.