- Preheat the oven to 320F (160C).
- Grease the 22*22cm (8.65*8.65 inch) baking tin with butter (or cover with baking paper).
- Melt the butter, and cool slightly.
- Peel bananas, and mash with a fork.
- Mix with butter, sugar and eggs.
- Separately combine flour, cocoa and baking powder, and sift over the liquid mass. Mix until smooth.
- Chop the chocolate, and combine with the dough.
- Pour the mass into the baking tin, flatten and bake for 30-35 minutes.