In a small bowl, mix the sugar and cinnamon for the topping.
1/2 tbsp sugar, 1/4 tsp ground cinnamon
Add a spoonful of the cake batter into a lightly greased mug, sprinkle with the cinnamon sugar, repeat the process until you are done with the cake batter. Make sure to finish with a generous layer of cinnamon sugar.
Microwave on high for 1 to 1 1/2 minutes. Start checking at the 1-minute mark to avoid overcooking. The cake should rise and be slightly springy to the touch.