Preheat oven to 350°. Prepare a 9"x13" pan with cooking spray; set aside. In a mixing bowl, combine cake mix, whipped topping mix, and baking powder. In another mixing bowl, combine half cup lemon juice, water, egg whites, oil, and vanilla extract. Mix dry ingredients with wet ingredients just until moistened. Spread mixture into prepared pan. Bake for 25 minutes or until golden brown. To prepare topping, combine sweetened condensed milk, egg yolks, remaining lemon juice, and lemon peel. Spread evenly over hot cake. Return to oven and bake for eight minutes more. Cool.