Preheat the oven to 350°F and generously spray three 8- or 9-inch round cake pans (or you can do this with one cake pan, baking one at a time) with cooking spray. Using a stand mixer fitted with the paddle attachment (or a large mixing bowl with a hand mixer), combine the butter, sugar, egg whites and lemon juice and beat on medium speed until creamy and free of lumps.
In a separate bowl, combine the flour, baking powder and salt. Mix well. Gradually add to the butter mixture and beat on medium speed until just combined. Add the lemon zest and the milk gradually and mix until creamy. Divide the batter between the prepared cake pans and bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out clean. Let cool in pan for 5 minutes, remove from pan and let cool completely on a wire rack. Using a serrated bread knife, cut the rounded tops off each cooled cake.
MAKE THE FROSTING
While the cakes are cooking and cooling, make the frosting. In a large bowl, mix (using whisk attachment) the butter with a hand-held or stand mixer on medium speed until fluffy. Add the powdered sugar one cup at a time until just combined.
Divide the frosting in half. Add the lemon juice, lemon zest and yellow food coloring to one portion and add the lime juice, lime zest and green food coloring to the other. Mix each until combined. Refrigerate until ready to assemble.
ASSEMBLE THE CAKE
Place one cake in the center of a cake or serving plate. Spread the frosting evenly and generously over the top in an even layer. Top with another cake and spread frosting over the top. Top with the remaining cake.
Using an offset or icing spatula, apply frosting to the sides of the cake and create a smooth surface. Repeat on the top of the cake.
Notes
Use your creativity as you frost this cake! You have two different frosting flavors and colors, so you can swirl the colors on the tops and sides or just the top. Or you could use one color for the sides and the other for the top.
For extra lemon flavor, add 1-2 teaspoons of lemon extract to the batter.
Crumb coat the cake and pop it in the fridge for 15 minutes first, if desired.
Reserve the egg yolks and fry them up for breakfast!
Adorn the sides of your cake with lemon or lime slices. Or make candied lemons to use as decor! (recipe below)