Melt dark chocolate and butter using a double boiler.
Add sugar to the melted mixture and mix well.
Add eggs one at a time, mixing thoroughly.
Sift in flour and cocoa powder, then mix until combined.
Line a square pan with parchment paper and pour in the batter.
Bake in a preheated oven at 170°C for 20–25 minutes.
Let cool completely, then cut into desired sizes.