Add dry ingredients to your largest mug and mix well. Add the egg and mix thoroughly. Pour the milk and oil and mix well. Add the chocolate chips (if using) and vanilla extract.
Put you mug in the microwave and cook for 3 minutes at 1000watts (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.