Preheat oven to 325 degrees F.
Add the cake mix and Diet Sprite to a large mixing bowl. Use a hand-held mixer on medium for 2 minutes.
Add cupcake liners to a pan, then fill half way with cake batter. You should get 24 cupcakes.
Bake 19 to 23 minutes. Let the cupcakes cool completely.
While the cupcakes are baking prepare the pudding. Add the pudding mix and 2 cups of skim milk to a bowl and whisk for 2 minutes. The pudding will thicken after about 5 minutes.
Remove the center of each cupcake being careful not to go all the way to the bottom. Save the top of each cupcake.
Put pudding in a piping bag (or a zip up bag with the bottom corner tip cut off) and fill the center of each cupcake.
Put the top of the cupcake back on. Frost the cupcakes with the fat-free Cool Whip. Enjoy!