In a small bowl, whisk egg whites, 0.25 cup of milk, and lemon zest; set aside.
In a mixing bowl, beat softened butter and sugar until creamy, about 2-3 minutes. Mix in dry ingredients, then gradually add egg white mixture and remaining milk; beat until combined.
Prepare cupcake tins with paper liners. Fill half full with batter and bake at 350°F for 15 minutes. Cool in the pan 5 minutes, then transfer to wire rack to cool completely.
For frosting, beat softened butter for 3-5 minutes until pale. Add powdered sugar, lemon zest and juice, then heavy cream; beat until fluffy.
Frost cooled cupcakes using a piping bag or spatula. Decorate as desired.
Notes
Best enjoyed fresh, but can be stored in an airtight container for up to 2 days.