Melt the butter in a 6 to 8-ounce mug using the microwave.
Add vanilla extract and milk. Stir.
Add all dry ingredients except chocolate chips. Stir well. Make sure no dry ingredients remain in the bottom of the cup.
Add chocolate chips and stir well again.
Microwave for 1 minute.
Check for doneness. The cake should spring back when lightly touched and no wet spots should remain. If the cake is not done, microwave for 15 more seconds.