You can use any basic boxed cake mix to do this. Any flavor is fine, just make sure to pay attention to later steps.
Step 2: Buy Soda
I recommend buying a 12 pack of canned DIET soda (to get that fat-freeness). To prevent any soda flavor from entering the cake, use Diet Sprite or something of similar type. However, if you are a chocolate cake fan, using Diet Coke or Pepsi would be fine, as the chocolate masks the Pepsi taste.
Step 3: Ignore!
Ignore the directions on the box to add oil, water, and/or eggs and instead just add ONE can of soda.
Step 4: Bake
Bake for the amount of time at the temperature of the box's instructions.
Step 5: Enjoy!
What you should have now is a delicious, fudgy cake, that is also fat free and does not need frosting, because of its sheer delciousness.