Melt the dark chocolate and butter using a double boiler.
Add sugar to the melted chocolate and butter, and mix well.
Add eggs and whisk thoroughly.
Gently fold in flour, cocoa powder, and salt until smooth.
Line a pan with parchment paper and pour in the batter.
Bake in the oven at 170°C for 25 minutes.
Let it cool and cut into desired sizes.