Melt dark chocolate and butter together in a double boiler.
Add sugar and mix well.
Add eggs one at a time, stirring thoroughly after each addition.
Sift flour and salt together, then mix into the batter.
Pour the batter into a pan and spread it evenly.
Bake in a preheated oven at 180°C for 20–25 minutes.
Let cool, then cut into serving pieces.