16 servings
Preheat the oven to 180°C (356°F).
Melt the butter and dark chocolate together in a double boiler.
Place sugar and eggs in a bowl and mix well.
Add the melted butter and chocolate, then mix.
Sift cake flour and cocoa powder into the mixture and combine.
Add walnuts and mix evenly.
Pour the batter into a mold.
Bake in the oven for 25 minutes.