Preheat the oven to 180°C (fan) and line a muffin tin with 12 cases.

In a bowl mix together the cocoa powder and 5 tbsp boiling water, beating to form a smooth paste. In a separate mixing bowl, mash the bananas, tip in the chocolate paste, then add the sugar, egg and oil, stirring thoroughly to combine. Finally stir in the flour and chocolate chips and spoon the mixture into the prepared muffin cases.

Bake in the preheated oven for 20 minutes until well risen and springy to touch. Remove muffins from the tin and cool on a wire rack.