Preheat the oven to 180°C (356°F).
Mash the bananas well.
Melt the dark chocolate and butter together in a double boiler.
Mix the eggs and sugar, then add the banana, chocolate, and butter mixture and combine well.
Sift in flour, baking powder, and salt, and mix evenly.
Pour the batter into a brownie pan.
Bake in the oven at 180°C (356°F) for 25~30 minutes.