In a medium sized mixing bowl mash the banana, then whisk in the egg
Mix in remaining ingredients until smooth
Pour into a large mug (or two smaller mugs) and heat in the microwave for 1 minute
Check then heat for another minute. Keep checking to make sure the mixture does not rise too high. If it does, stop the microwave and open the door. Once the mixture has sunk, continue cooking.
Remove from microwave, tip onto a plate, top and enjoy
You can also eat the mug cake straight from the mug.