Cake :
Preheat the oven to 350 F (180 C).
Grease two 9-inch (23 cm) round cake pans with removable sides. Reserve.

In the bowl of a stand mixer, fitted with the whisk attachment, mix the sugar, butter, lemon zest, lemonade, and vanilla extract until well combined (about 5 minutes).
Add the eggs and egg whites, one at a time, beating well after each addition.
Add the flour, baking powder, baking soda, and salt, which have been sifted together, alternating with the buttermilk. Mix well.

Divide the batter evenly between the two prepared pans and tap the pans on the counter to remove any air bubbles.
Bake for 20 to 25 minutes, or until a toothpick inserted in the center comes out clean.
Let cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.

Glaze :
In the bowl of a stand mixer, fitted with the whisk attachment, beat all the ingredients, except the powdered sugar, until smooth.
Add the powdered sugar, a little at a time, beating at low speed, until well combined. Refrigerate for 1 hour.

Place one cake layer on a serving plate.
Spread 1/2 cup (125 ml) of glaze over the top, using a spatula, then place the second cake layer on top, straightening the sides.
Cover the entire cake with the remaining glaze.
Store the cake, covered with a dome, in the refrigerator.