Using a fork, mashed the peel banana. Meanwhile, melt the margarine in a mug in the microwave.
In a bowl, combine the flour, baking powder, cinnamon, mashed banana, margarine, and milk. Finally, fold in the chocolate chips.
Pour the batter into a microwave-safe mug and bake for 60-90 seconds at 800 watts. For best results, cook the banan mug cake in 30-second intervals, check in between to see if it's fully set.