Melt dark chocolate and butter together over a double boiler.
Add sugar and mix well.
Add eggs one at a time, whisking after each addition.
Sift in cake flour, cocoa powder, and salt, then gently fold into the batter.
Add walnuts or any nuts of your choice.
Pour the batter into the baking pan.
Bake at 180°C (356°F) for 25 minutes.
Let cool, then cut into bite-sized pieces.