Prepare and bake the cake according to the package directions for a 9×13 inch pan.
Let the cake cool in the pan for 15 minutes after removing it from the oven.
While the cake cools, prepare the Jello according to the package directions, but use 1 cup boiling water and 1 cup cold water instead of the amounts listed on the Jello box. Stir until the gelatin is completely dissolved.
Use the handle of a wooden spoon or a large fork to poke holes all over the top of the warm cake, spacing the holes about 1 inch apart. Poke all the way down to the bottom of the cake.
Slowly pour the liquid Jello mixture evenly over the entire surface of the cake, allowing the liquid to soak into the holes.
Refrigerate the cake for at least 3 to 4 hours, or until the Jello is completely set.
Once the Jello is set, spread the thawed whipped topping evenly over the top of the cake.
Chill for an additional 30 minutes before slicing and serving.
Notes
For a colorful effect, use different colored Jello flavors and poke holes in sections, pouring one color into each section.
You can substitute the whipped topping with homemade stabilized whipped cream.
For a richer flavor, use milk instead of water when preparing the Jello, following the package liquid ratio.