Preheat oven to 350 F. Spray 2 8-inch round cake pans.
Whisk together the cake flour, baking powder, pink lemonade powder, and salt. Set aside.
Whisk together the buttermilk, egg whites, and vanilla in a medium bowl. Set aside.
Rub the lemon zest into the sugar with your fingers, sugar will be fragrant.
In mixer with paddle attachment, or with a hand mixer, beat butter and lemon-sugar at medium speed for a full 3 minutes, until the mixture is very light.
Reduce speed to low and gradually add half of the dry ingredients and half of the wet ingredients. When combined add the remaining dry and wet ingredients. Beat until combined, scrape down side of bowl if needed. Beat two minutes more on medium speed to aerate the batter.
Divide the batter between the two pans and smooth the tops with a rubber spatula.
Bake for 30-35 minutes, or until the cakes are well risen and springy to the touch – a thin knife inserted into the centers should come out clean.
Transfer the cakes to cooling racks and cool for about 5 minutes, then remove from pans, peel off parchment, and let cool completely.
Make the Frosting
Beat butter in a mixer until smooth and creamy.
With mixer on lowest speed, slowly and gradually add in the powdered sugar.
Add in frozen pink lemonade concentrate, vanilla, and lemon zest. If you are using the food coloring, you can add that now as well.
Beat on medium speed until creamy and well incorporated.
One medium lemon will yield approximately one teaspoon of zest. After zesting the lemons, store the lemon juice in the freezer to use later in other recipes like our Strawberry Lemonade.
The drop of red food coloring is optional, but can enhance the pink color of the frosting if you like.