1 box of regular or low-fat cake mix
1 12-ounce can of diet Coke
Use any regular or low-fat cake mix and add the diet Coke. Bake the cake according to the package directions.
You can frost the cake a couple of different ways... Sprinkle the cake with confectioners sugar, or mix together an 8-ounce tub of Cool Whip and 1 package of sugar-free, fat-free pudding mix (any flavor appropriate to cake flavor).
diet Coke/Chocolate Cake Mix
diet Sprite/White Cake Mix
diet Cherry Coke/Cherry Cake Mix