Preheat oven to 350 degrees F (175 degrees C). Line cupcake tin with paper liners.

In a bowl, whisk together flour, baking powder, and salt. In another bowl, beat butter and sugar until creamy. Add egg whites, lemon zest, and lemon juice; mix well.

Gradually add dry ingredients to wet, alternating with milk, mixing until just combined.

Fill cupcake liners 2/3 full and bake for 15 minutes, or until a toothpick inserted into the center comes out clean. Cool completely on wire rack.

For the frosting, beat powdered sugar and heavy cream until fluffy. Spread or pipe onto cooled cupcakes.