Preheat the oven to 180°C (356°F).
Melt dark chocolate and butter in a double boiler.
Add sugar to the melted chocolate and mix well.
Add eggs and whisk thoroughly.
Sift cake flour, cocoa powder, and salt into the mixture and blend.
Pour the batter into a pan and smooth the top.
Bake in the oven at 180°C (356°F) for 20-25 minutes.
Let it cool, then cut into bite-size pieces.