Preheat the oven to 180°C (356°F).
Mash the banana with a fork.
In a bowl, mix the mashed banana, egg, and sugar together.
Add the melted butter and milk, mixing well.
Sift in the cake flour, cocoa powder, and baking powder and gently combine.
Finely chop the chocolate and fold into the batter.
Pour the batter into a mold and level the surface.
Bake in the oven at 180°C (356°F) for 20–25 minutes.