Preheat the oven to 180°C (356°F).
Melt dark chocolate and butter together in a double boiler.
Add sugar to the melted chocolate mixture and mix well.
Add eggs one at a time, mixing each in thoroughly.
Sift cake flour, cocoa powder, and salt into the mixture and combine.
Pour the well-mixed batter into a baking pan.
Bake in the oven for 20-25 minutes.
Let cool and cut into desired serving sizes.