Slice the banana thinly.
Melt dark chocolate and butter together in a double boiler.
Add sugar and eggs to the melted chocolate and mix well.
Sift cake flour and cocoa powder into the mixture and combine evenly.
Gently fold the sliced banana into the batter.
Pour the batter into a pan lined with parchment paper and place banana slices on top.
Bake in a 180°C oven for 25-30 minutes.
Let cool and cut into bite-sized pieces.