Saw this on one of the forums:
1 Coffee Mug
4 tablespoons flour(that's plain flour,not self-rising)
4 tablespoons sugar
2 tablespoons baking cocoa
3 tablespoons milk
3 tablespoons oil
3 tablespoons chocolate chips (optional)
Small splash of vanilla
Add dry ingredients to mug, and mix well . Add the egg and mix
thoroughly.Pour in the milk and oil and mix well.
Add the chocolate chips (if using)and vanilla, and mix again.
Put your mug in the microwave and cook for3 minutes at 1000 watts.
The cake will rise over the top of the mug,but don't be alarmed!
Allow to cool a little, and tip out onto a plate if desired.