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.
Video
Notes
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.