Mix all the ingredients into melted chocolate and microwave for 1 minute 30 seconds.
Melt dark chocolate and butter using a double boiler.
Add sugar and mix well.
Add eggs one at a time and gently stir.
Sift cake flour and cocoa powder into the mixture and combine.
Add walnuts or other nuts if desired.
Pour the batter into a pan and smooth it out evenly.
Bake in a preheated oven at 180°C for 20 to 25 minutes.