Bake cake according to package directions in 2 cake pans. Cool.
When cool, slice each layer in half horizontally, making 4 layers.
In a medium mixing bowl, combine sweetened condensed milk, lemon juice and lemon zest. Reserve half of mixture and set aside.
Using half of the lemon milk mixture, pour between each cake layer.
Gently fold the whipped topping into reserved lemon milk mixture. Using this as the frosting, frost the entire cake.
Chill overnight in the refrigerator before serving.