In a mug, mash the banana then add the rest of the ingredients and whisk together with a fork until well combined.
Microwave on 700w for 60 seconds. It might be differe°nt from one microwave to another, but stop cooking when the edges set and the center is still a bit wet and shiny. If you over bake the batter, you will get just a regular chocolate cake.
If you don't want to bake this molten cake in the microwave, you can just bake it in the oven for around 10 minutes on 200C.