Found from acexkeikai via tammylee; 



I used a bowl instead because I was scared of making a mess. I also didn't have chocolate chips so I broke-up a bit of a chocolate bar [hence the peanuts].

RECIPE:
4 tablespoons flour
4 tablespoons sugar
2 tablespoons cocoa
1 egg
3 tablespoons milk
3 tablespoons oil
3 tablespoons chocolate chips (optional)
a small splash of vanilla extract
1 large coffee mug

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 extract, and mix again.
Put your mug in the microwave and cook for 3 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.

It would defiantly need to be served with syrup or icecream/frozen yogert, as it is rather dense/dry.