Preheat the oven to 180°C (356°F).
Melt dark chocolate and unsalted butter together over a double boiler.
In a bowl, whisk together eggs and sugar.
Add the melted chocolate and butter mixture to the egg mixture and mix well.
Sift in cake flour, cocoa powder, and salt, then mix until combined.
Pour the batter into a baking pan and smooth the surface.
Bake in the preheated oven for 25 minutes.
Let it cool completely, then cut into bite-sized pieces.