Mix all ingredients together into an oversized mug. This cake rises a lot so if you don't have a very large mug, I suggest you split the batter into two mugs after mixing.
Mix with a small whisk until no lumps remain.
For one oversized mug, microwave for 1 1/2 minutes. Cake should have a dry surface and knife inserted should come out clean. You may want to reduce the time for two smaller mugs.
