Interesting factoid: You can, technically, make chocolate cake in a coffee mug.
Another factoid: You shouldn't.
I found this recipe on http://baconconcentrate.blogspot.com/ via the serious eats photograzing site (highly recommended). I decided to try it out, but didn't take picture, because, honestly, it sucked. I could barely taste any chocolate at all, and it was freakishly dense. It was like eating a brick-sponge hybrid. That didn't taste like chocolate. Here's the recipe anyway! You love it.
4 tablespoons flour
4 tablespoons sugar
2 tablespoons cocoa
3 tablespoons milk
3 tablespoons oil
A small splash of vanilla extract
1 large coffee mug
Mix dry ingredients in the coffee mug, then add milk, oil, egg and vanilla. Mix together and microwave for 2.5 minutes. Take what you will from this experience.