Preheat the oven to 180°C (356°F).
Place dark chocolate and unsalted butter in a bowl and melt over a double boiler.
Add sugar and salt, and mix well.
Add eggs one at a time, whipping thoroughly after each addition.
Sift in gluten-free cake flour and cocoa powder, then mix until well combined.
Fold in walnuts.
Pour the batter into a pan lined with parchment paper.
Bake in the oven at 180°C for 25 to 30 minutes.
Let cool, then cut into bite-sized pieces.