With a mixer, beat first 5 ingredients until well blended.
Add eggs and egg whites; beat well. Combine flour, baking powder, baking soda, and salt. Add flour mixture and buttermilk alternately to egg/sugar mixture. Beat well after each addition.
Preheat oven to 350 F. Pour batter into two 9-inch round cake pans coated with cooking spray. Bake 20 minutes or until toothpick comes out clean.
Cool in pans 10 minutes, then remove. Cool completely on wire rack before frosting.
For frosting, place cream cheese, grated lemon rind, and lemonade concentrate in a large bowl. Beat on high until fluffy. Add powdered sugar and beat on low until just blended.
Place one cake layer on intended serving tray; spread with 1/2 cup frosting. If frosting is too runny, refrigerate for an hour and try again. Top with second cake layer. Spread remaining frosting on top, letting icing run down the sides, if desired.
Decorate with candied lemon slices, if desired. Store cake loosely covered in the refrigerator until ready to serve.
Per serving: 415 calories, 11 grams fat (7 grams saturated fat),
73 grams total carbs, 1 gram fiber, 7 grams protein