Preheat the oven to 180°C (356°F).
Melt the dark chocolate and butter using a double boiler.
In a bowl, add the sugar and eggs, and mix well.
Combine the melted chocolate and butter mixture with the egg mixture.
Sift in the flour and cocoa powder, and mix until incorporated.
Add walnuts or other nuts if desired.
Pour the batter into the pan.
Bake in the oven for 25-30 minutes.