Melt dark chocolate and butter together in a double boiler.
Add sugar and mix well.
Add eggs one at a time, beating after each addition.
Sift cake flour and cocoa powder into the mixture and stir to combine.
Add walnuts or your choice of nuts.
Pour the batter into a baking pan.
Bake in a preheated oven at 180°C (356°F) for 20-25 minutes.
Let cool and cut into bite-sized pieces.