Melt dark chocolate and butter in the microwave.
Combine sugar and eggs in a bowl and mix.
Add the melted chocolate and butter, then mix again until smooth.
Sift in flour and cocoa powder, then blend together.
Finely chop nuts and banana, then fold into the batter.
Pour the batter into a mold and spread it evenly.
Bake in a 180°C oven for 25 minutes.