Melt butter and dark chocolate together using a double boiler.
Add sugar to the melted chocolate mixture and mix well.
Add eggs and stir thoroughly.
Sift flour and cocoa powder into the mixture and blend evenly.
Pour the batter into a baking tray and spread it evenly.
Bake at 170°C for 25 minutes in a preheated oven.
Let it cool completely, then cut into bar shapes.