Bake the banana in its skin for 10 minutes, or until the skin is black.
Meanwhile, warm the syrup in a small saucepan over a medium heat for 2-3 minutes, or in a medium-low microwave for 1 minute until very runny. Stir in the cocoa powder until smooth and chocolate-like. Keep warm.
When the banana is cooked, discard the skin, put the flesh on a plate, pour the chocolate sauce over and serve.