Preheat the oven to 180°C (356°F).
Melt dark chocolate and butter together using a double boiler.
In a bowl, mix eggs and sugar.
Add the chocolate-butter mixture to the eggs and stir to combine.
Sift flour and cocoa powder into the batter and mix well.
Add walnuts and gently fold them in.
Pour the batter into a pan and spread evenly.
Bake in the oven for 25 minutes.