Melt dark chocolate and butter together using a double boiler.
Add sugar to the melted chocolate and butter mixture and mix well.
Sift in flour and cocoa powder, then mix until combined.
Add eggs and gently blend to form a smooth batter.
Pour the batter into a square pan and spread evenly.
Bake at 180°C (356°F) for 20-25 minutes.
Let cool, then cut into pieces and serve.