CAKE: Cake: Preheat oven to 350 degrees. Place granulated sugar, butter, lemon zest, lemonade concentrate and vanilla in a large bowl; beat with a mixer at medium speed until well blended (about 5 minutes). Add eggs and egg whites, 1 at a time, beating well after each addition. In a separate bowl, combine flour, baking powder, salt, and baking soda; stir well with a whisk. Add flour mixture and buttermilk alternately to sugar mixture, beginning and ending with flour mixture; beat well after each addition.
Pour batter into 2 (9-inch) round cake pans coated with cooking spray; tap pans once on counter to remove air bubbles. Bake for 20 minutes or until wooden pick inserted in center comes out clean. Cool in pans 10 minutes on a wire rack; remove from pans. Cool completely on wire rack.
FROSTING: Frosting: Place butter, lemon zest, lemonade concentrate, vanilla and cream cheese into a large bowl; beat with a mixer at high speed until fluffy. Add confectioners' sugar, and beat at low speed just until blended (do not overbeat). Chill 1 hour.
Place 1 cake layer on a plate; spread with 1/2 cup frosting. Top with remaining cake layer. Spread remaining frosting over top and sides of cake. Store cake loosely covered in the refrigerator.