In a large microwave-safe mug, mash the banana until mostly smooth.
Whisk in sugar, milk, oil, and vanilla until well combined.
Add flour, baking powder, cinnamon, and salt. Stir until no dry streaks remain. Fold in chocolate chips and walnuts if using.
Microwave on High for about 2 minutes, or until the top is set and springs back lightly. If underdone, microwave in 0.15-minute bursts.
Let stand 1 minute to cool slightly. Enjoy warm straight from the mug.
Notes
Microwaves vary—start checking at 1.5 minutes. For gluten-free, use a 1:1 baking blend. For dairy-free, use plant-based milk and dairy-free chips. Add a sprinkle of coarse sugar on top for crunch.