Preheat oven according to box directions. Spray 9x13 pan with cooking spray.
Beat cake mix, diet soda and egg whites in a large bowl for 2 minutes. Pour mixture into prepared pan.
Decrease cooking time by 5 minutes from what the box says. A toothpick inserted in the middle of the cake will come out clean when it is finished.
Pull cake out from oven and let cool for 15 minutes. Using a large fork, ice pick or toothpick poke large holes into cake about every 1/2 inch of the cake. (Don't worry about how it looks , we'll be covering it up with frosting!)
Mix together Jell-o and boiling water and stir for 2 minutes or until dissolved. Add in cold water. Pour over poked cake. Refrigerate until cooled completely.
Combine pudding and 1-1/2 cups cold milk. Beat together until thickened, about 2-3 minutes. Fold in desired amount of Cool Whip (I used about half a container). When it's the consistency and taste desired, spread over cooled cake. Slice up, serve and enjoy! Make sure to keep cake chilled in refrigerator!