In a large sized (10 oz.) mug, mix all cake ingredients until just combined. In another small bowl mix together brown sugar, cinnamon powder and melted butter, make a swirl while pouring the cinnamon mixture over the cake batter.
Microwave on high for 90 seconds to 2 minutes, watching to make sure it doesn’t spill over (depending on the size of the mug).
Let cool one minute before eating. Top with whipped cream - optional.