Preheat the oven to 170°C (338°F).
Melt the dark chocolate and butter together using a double boiler.
Add eggs and sugar to a bowl and mix well.
Pour the sugar and egg mixture into the melted chocolate mixture and combine.
Sift in flour, cocoa powder, and salt; mix until just combined.
Pour the batter into a baking pan and spread it evenly.
Bake in the oven for 20–25 minutes.
Let it cool completely, then cut into desired sizes.