Prepare the Mug: Use a microwave-safe mug. Lightly grease the inside with a little bit of oil or butter to prevent sticking.
Mash the Banana: In a small bowl, mash the ripe banana with a fork until smooth.
Mix Dry Ingredients: In the mug, combine the flour, brown sugar, baking powder, cinnamon, and salt. Stir well.
Add Wet Ingredients: Add the mashed banana, milk, vegetable oil, and vanilla extract to the mug. Stir until all the ingredients are well combined and there are no lumps.
Microwave: Place the mug in the microwave and cook on high for about 1.5 to 2 minutes. The exact time may vary depending on your microwave's power, so start with 1.5 minutes and check for doneness. The cake is done when it has risen and is firm to the touch.
Cool Slightly and Enjoy: Let the mug cake cool for a minute or two before eating. Enjoy it warm directly from the mug, or you can turn it out onto a plate.
