In a microwave-safe mug (roughly 12 oz capacity), mash the ripe banana with a fork until smooth.
Add the all-purpose flour, granulated sugar, baking powder, ground cinnamon, and a pinch of salt to the mashed banana. Stir well to combine.
Add the vegetable oil, milk, and vanilla extract to the mug. Mix thoroughly until the batter is smooth and all ingredients are incorporated.
Microwave the mug on high for 90 to 120 seconds. Check for doneness by inserting a toothpick into the center; it should come out clean or with a few moist crumbs.
If the banana bread is not fully cooked, microwave in additional 10-second intervals until done, being careful not to overcook.
Let the banana bread cool for 2 minutes before eating directly from the mug.