Melt dark chocolate and butter over a double boiler.
Add sugar to the melted chocolate mixture and mix well.
Add eggs and stir until well combined.
Sift cake flour and cocoa powder into the mixture and gently mix until smooth.
Pour the batter into a baking pan and spread evenly.
Bake in a preheated oven at 180°C for 20-25 minutes.
Cool completely and cut into bite-sized pieces.