1 box cake mix (any flavor)
10-12 fl oz diet soda (I prefer flavored seltzer water because it doesn't have any artificial sweeteners)
Mix together and bake according to the time and temperature on the box. Top with light whipped topping. For real, that's it.
A co-worker told me about this, and I was definitely skeptical but figured it was worth a try. I would think it would need the eggs and oil to come together, but it really does work. Think of all the fat that saves.
I've made this recipe twice now (dark chocolate fudge + coke zero, and french vanilla + raspberry seltzer). The first one I made in cupcakes (btw, don't fill the cups all the way) and the second as a cake.
If I could find a vegan cake mix, I'd be interested to see how it works without all the oil.