Prepare and bake the white cake according to the package directions using a 9×13 inch baking pan.
Let the cake cool in the pan for about 15 minutes after removing it from the oven.
While the cake cools slightly, prepare the Jello according to the package directions, but substitute the cold water amount with 1/2 cup cold water. Dissolve the gelatin completely in the boiling water, then stir in the cold water.
Use the handle of a wooden spoon or a large fork to poke holes all over the top of the warm cake, spacing them about 1 inch apart. Poke all the way down to the bottom of the cake.
Slowly pour the prepared liquid Jello mixture evenly over the entire surface of the cake, allowing the liquid to seep into the holes.
Refrigerate the cake for at least 3 to 4 hours, or until the Jello is completely set.
Once set, spread the thawed whipped topping evenly over the top of the cake before slicing and serving.
Notes
For a Christmas Jello Poke Cake, use red and green Jello flavors and layer them if desired.
For a rainbow effect, use smaller cakes or divide the batter and use different colored Jello flavors in separate batches.
You can substitute the whipped topping with a cream cheese frosting for a richer topping.