4 tablespoons flour(that's plain flour, not self-rising)           

4 tablespoons sugar
2 tablespoons baking cocoa
1 egg

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 for 3 minutes on 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.
EAT! (this can serve 2 if you want to share!)