Melt dark chocolate and butter together in a double boiler.
Add sugar to the melted chocolate mixture and stir.
Add the eggs and mix until well combined.
Add flour and salt, mixing to form a batter.
Fold in coarsely chopped walnuts.
Pour the batter into a baking pan and spread it evenly.
Bake in a preheated oven at 180°C for 25 minutes.
Let it cool slightly, then cut into pieces to serve.