Preheat the oven to 180°C (356°F).
Mash the bananas and set aside.
Melt dark chocolate and butter together in a double boiler.
Combine the mashed bananas, melted chocolate, sugar, and eggs.
Sift cake flour in and mix until well combined.
Pour the batter into a baking pan.
Bake in the oven for 25–30 minutes.