Preheat oven to 350 degrees F. Spray the bottom of a 9x13 baking dish with nonstick cooking spray. Mix cake mix according to package directions.
Pour into prepared baking dish and bake for 30 minutes or until a toothpick inserted into the middle comes out clean.
In a small bowl, whisk lime gelatin with one cup of boiling water until completely dissolved. Then whisk in the 1/2 cup of cold water.
Remove the cake from the oven and allow to cool for 5 minutes. With the end of a wooden spoon, poke holes into the cake about 2 inches apart.
Slowly drizzle the gelatin over the entire surface of the cake so that it is covered and drips into the holes. Refrigerate for at least two hours.
Then, spread the Key Lime Creme Pie topping over the cake and then cover completely with whipped topping.
Return to refrigerator to cool for at least one hour before serving.
Decorate with fresh lime zest and lime slices, if desired.
Notes
Leftover cake should be covered tightly and stored in the refrigerator for 3 days. To freeze, cover with plastic wrap then with a layer of foil wrap. Cake can be frozen for 3 months. To defrost, place in refrigerator for several hours.
If you can't find key lime pie filling, just leave it out. Or for more of a lemon/line flavor, try using lemon pie filling instead.