Melt dark chocolate and butter together in a double boiler.
Add sugar and mix well.
Beat in the eggs one at a time.
Sift in flour and cocoa powder, then mix together.
Add walnuts and fold into the batter.
Pour the batter into a parchment-lined pan.
Bake in a preheated oven at 180°C (356°F) for 20 minutes.