If using the oven preheat it to 200 degrees Celsius.
Combine banana and milk in a bowl.
Combine chocolate protein powder, oat flour, xylitol, baking soda and salt in a separate bowl.
Combine the dry and wet ingredients.
Grease a mug or small bowl using coconut oil or cooking spray.
Pour the batter to the bowl and run go highest power in the microwave for 1-2 minutes, or bake in the oven for 15-20 minutes, depending on how fudge you prefer your mug cake.