In a microwave-safe mug (10 to 12 oz), mash the banana until mostly smooth.
Whisk in the egg, sugar, melted butter, milk, and vanilla until fully combined.
Add flour, baking powder, cinnamon, and salt. Stir until no dry streaks remain, scraping the bottom and sides of the mug.
Fold in chocolate chips (if using) and smooth the top.
Microwave on high for 75 to 90 seconds, until the top is set and springs back lightly. Do not overcook; the center should be just set.
Let rest for 1 minute to cool slightly, then enjoy straight from the mug.
Notes
Microwave wattages vary—start with 75 seconds and add 10-second bursts if needed. For a lighter cake, swap butter for 1 tbsp neutral oil. Add-ins like chopped walnuts or a pinch of nutmeg are delicious.