In a small bowl, mash the ripe banana with a fork until smooth.
Add the all-purpose flour, granulated sugar, baking powder, baking soda, ground cinnamon, and a pinch of salt to the mashed banana. Stir to combine.
Add the vegetable oil, milk, and vanilla extract to the bowl. Mix until the batter is smooth and well combined.
Lightly grease a microwave-safe mug or small bowl.
Pour the batter into the prepared mug or bowl.
Microwave on high for 90 to 120 seconds, or until the muffin has risen and is set in the center. Cooking time may vary depending on microwave wattage.
Carefully remove from the microwave and allow to cool for 1-2 minutes before eating.