Mash the banana in a small bowl and mix in oil and vanilla extract.
Put the flour, sugar (if using), baking powder and salt in a small espresso mug. Blend thoroughly with a fork.
Add the mashed bananas, mix in any additional mix-ins like chocolate chips, nuts, etc.
Microwave 90 seconds then let it cool about 1 minute before eating. If making a double batch in a large mug, cook 2 minutes.
Oven: If you don't have a microwave, you can bake this in a preheated oven in a ramekin of cupcake liner. Bake until the tops are golden and a toothpick inserted in the center comes out clean, 325F for 25 to 30 minutes.Air Fryer: If you want to bake this in the air fryer, preheat the air fryer to 300°F. Bake until the tops are golden and a toothpick inserted in the center comes out clean, about 15 minutes. Let cool before eating.See trouble shooting notes above!