Prepare and bake cake according to directions. You can do a 9x13 pan or even a 2-layer cake. Both work great here.
Add boiling water to the Jell-o. Stir until dissolved.
Poke cake with a fork to make lots of little holes.
Pour Jell-o onto the cake.
Refrigerate. When cool, frost with Cool Whip. Keep refrigerated and serve cold.