Add all the dry ingredients (flour, sugar, cocoa) into a mug and mix well. Add the egg and mix thoroughly. Pour in the milk and oil and mix well. Add the chocolate chips and vanilla extract and mix again.
Put the mug in the microwave and and cook for 3 minutes at 1000 watts (high). The cake will rise over the top of the mug, but don't be alarmed. Allow to cool a little and tip out if desired.
EAT! This can serve two people, if you want to feel slightly more virtuous.