Chop the banana in half. Slice one half into small rounds (these are what you'll use to top the oatmeal) and place the other half in a small saucepan. Mash the banana in the saucepan with a potato masher until it's completely soft and broken down.
Stir the oats, skimmed milk, water, zero-calorie sweetener and cocoa powder into the saucepan with the banana.
Place the pan over a medium-high heat and bring the mixture to a boil.
When boiling, reduce the heat down to medium-low and leave to simmer for about 5 minutes, until thick. You'll have to stir the oatmeal non-stop to make sure it doesn't stick to the bottom of the saucepan and burn.
When the oatmeal is soft and thick, pour it into a serving bowl. Layer the sliced banana in a line in the middle of the bowl. Sprinkle the chia seed on one side of the bananas and sprinkle the chopped chocolate pieces on the other side.