Add dry ingredients to large coffee mug and mix well. Add the egg and mix thoroughly. Pour in the milk and oil; mix well. Add the chocolate chips (if using) and vanilla extract; mix again. Put your mug in the microwave 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 ontn a plate if desired. Serves 2.