Preheat your oven to 350°F (175°C). Line a 24-cup muffin tin with paper cupcake liners.

In a small bowl, whisk together the egg whites, 0.25 cup of whole milk, and lemon zest. Set aside.

In a large mixing bowl, using an electric mixer, beat 0.75 cup of softened unsalted butter and granulated sugar until light and creamy, about 2-3 minutes.

In a separate bowl, combine the cake flour, baking powder, and kosher salt. Gradually add the dry ingredients to the butter mixture, mixing until just combined.

Slowly add the egg white mixture to the batter, then beat in the remaining 0.5 cup of whole milk until smooth. Do not overmix.

Fill each cupcake liner about half full with batter. Bake for 15 minutes, or until a toothpick inserted into the center comes out clean and the tops spring back when lightly touched.

Remove from oven and let the cupcakes cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.

While the cupcakes cool, prepare the frosting. In a large mixing bowl, beat 1 cup of softened unsalted butter with an electric mixer for 3-5 minutes until it becomes pale in color and fluffy.

Gradually add the powdered sugar, lemon zest, lemon juice, and heavy cream. Beat on low speed until combined, then increase to high speed and beat for another 3-5 minutes until the frosting is light and fluffy.

Once the cupcakes are completely cool, transfer the buttercream to a pastry bag fitted with your desired tip (or use an offset spatula) and pipe or spread the frosting generously onto each cupcake. Enjoy!