Peel and mash the banana.
Melt dark chocolate and butter using a double boiler.
Combine the mashed banana, melted chocolate, sugar, and egg.
Add cake flour and baking powder, and mix until well combined.
Pour the batter into a brownie mold.
Bake in a preheated oven at 180°C for 25 minutes.
Allow to cool, then cut into bite-sized pieces.