Prepare the cake mix as directed on the box, and bake in a 9x13” baking dish.
Let the cake cool for 5 minutes and then, using the end of a spatula or butter knife, poke holes in the cake, about ½” apart.
Combine the package of lemon Jell-O with the hot water and stir until dissolved, about 1 minute.
Add the cold water and stir until combined, about 1 minute.
Pour the Jell-O mixture over the warm cake. Let cool completely, about 4 hours.
Once cooled, spread the Cool Whip over the cake.
Garnish with lemon slices or zest, as desired. Slice and enjoy!
Store leftovers in the refrigerator.
*Note: for the hot water, I place it in a glass measuring cup and microwave for 90 seconds. Heat in 30-second increments until you see the water start to boil. Then combine with the Jell-O package immediately.