Preheat the oven to 180 degrees C. Spray a 9x5 inch loaf pan with non stick baking spray.
Mash the bananas in a large bowl. Whisk in the melted butter, brown sugar, egg and vanilla extract. Sift in the bi carbonate of soda, salt, flour and cocoa powder and stir mixture with a wooden spoon just until incorporated. The gently stir in the chocolate chips.
Pour the batter into the prepared pan and evenly sprinkle the chocolate chips. Bake for 50-55 minutes, or until a toothpick inserted in the centre comes out clean. Cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
Keep bread wrapped tightly at room temperature for up to 4 days.