Preheat the oven to 180°C (356°F).
Mash the banana with a fork.
Melt dark chocolate and butter together using a double boiler.
In a bowl, combine eggs and sugar and mix well.
Add the melted chocolate and butter mixture, stir well, then add the mashed banana.
Sift in cake flour, cocoa powder, and baking powder, and mix until well combined.
Pour the batter into the mold and smooth the surface.
Bake in the oven at 180°C (356°F) for 20–25 minutes.
Cool, then cut into appropriate sizes.