In a small bowl, add all cake ingredients. Mix with a whisk until batter is smooth. It will be thick.
In another small bowl, add cinnamon and sugar topping ingredients. Whisk until evenly mixed.
With a large spoon, place about half of the cake batter into a microwave-safe mug. Sprinkle half of the cinnamon sugar topping over the batter in the mug. Then add the other half of the cake batter to the mug. Sprinkle remaining cinnamon sugar topping evenly across the top of the cake.
Microwave cake at full power for about 1 minute or until cake is cooked. Let cool a few minutes before eating. Cake is best enjoyed shortly after it is cooked.