I've pinched this recipe from the Mathilda's Market blog...last week I tried it and it is DIVINE!!!
(the only secret is to measure properly)
5 MINUTE CHOCOLATE MUG CAKE
4 tablespoons flour
3 tablespoons sugar
2 tablespoons cocoa
3 tablespoons milk
3 tablespoons oil
3 tablespoons chocolate chips
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 2 1/2 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 onto a plate if desired.