Mash the bananas (I used a hand blender, but you can also mash with a fork) and mix together with the egg.
Mix the rest of the ingredients and a pinch of salt into the banana-egg mixture and mix with a whisk or spatula until combined. Divide between two oven-proof/microwave safe mugs.
OVEN: Bake for 20-25 minutes at 180 °C/350 °F (convention).
MICROWAVE: Bake for 140 seconds (2 minutes and 20 seconds) at 800 watts.
If desired, serve with a scoop of yogurt and some slices of banana or other fresh fruit.
Tips
I usually use 250 - 300 ml mugs for my mug cakes.
How to store
The mug cake is best when it comes out of the microwave and is still warm. As it cools, it dries out and is not nearly as tasty.