Bake a lemon cake as per box directions in 9X13 pan.
While it is baking, thaw a 6 oz can of frozen lemonade concentrate (If you can't find a 6 oz, use part of a large can.)
Mix thawed lemonade concentrate with 1 cup powdered sugar.
Poke holes in top of cake while warm with a wooden spoon. Pour lemonade
concentrate/powdered sugar mix over warm cake and let it cool
completely (I chilled mine in the fridge.)
Frost with cool whip and garnish with slices of fresh lemon. This is
best when kept chilled. Very refreshing! Pretty tart but very tasty!