Mix the dry cake mix with 10-12 oz of diet soda. (Do not use the egg or oil listed on the box).
Spray a 9 x 12" cake pan (or cupcake tin) with non-stick spray and bake according to package instructions.
While some people believe an entire can of pop makes the cake too moist, 10 oz seems to be the least I've seen suggested, so experiment yourself to find the best amount for your taste.
Tip: make sure to let the cake cook completely before you try to cut it. Because you aren't adding eggs, the cake will be tender and can tear if you cut it while it's warm. Some people have started adding 2 egg whites to the mix and report that the cake is firmer and rises better.
Note: based on a recipe from Weight Watchers.
There are 113 calories in 1 serving of Diet Coke Cake.