Ok here's one that has been around a bit, I finally tried it but added my own twist.
Coffee Cup Choc Cake
4 T flour , 3T milk
5 T sugar , 3 T oil
3 T cocoa, 1 T vanilla
1 egg, 1 T cinnamon
Cooking Spray, 1 Lg coffee mug
In a small bowl mix dry ingredients, add egg and blend completly. Add in milk and oil, mix well. Lastly add vanilla and mix one last time. Spray your mug with a little cooking spray.
Pour the mixture into your largest mug, put in microwave and cook for about 2 1/2 minutes.
This cake will rise to the very top and might even spill over a little (mine stayed in cup though). Don't worry it will be fine.
Allow it to cool a bit and tip out onto plate.
I cut mine three ways and added a scoop of vanilla ice cream to each one, the kids loved it and I didn't even have to turn on the oven!