In a small bowl or directly in a microwave-safe mug, mash the banana until mostly smooth.
Add the flour, sugar (if using), baking powder, salt, oil (if using), and vanilla extract to the mug with the banana. Stir with a fork until well combined and no dry clumps remain.
Stir in any optional mix-ins like chocolate chips, nuts, or sprinkles.
Microwave on high for 90 seconds. Cooking time may vary slightly depending on your microwave's wattage. Let it cool for about 1 minute before enjoying.
Notes
This mug cake is best enjoyed immediately. For a richer flavor, use a very ripe banana.