Melt dark chocolate and butter together in a double boiler.
In a bowl, whip the eggs and sugar together.
Mix the melted chocolate mixture into the eggs.
Sift flour and cocoa powder, add to the bowl, and gently mix.
Pour the batter into a pan lined with parchment paper.
Bake in a preheated oven at 180°C (356°F) for 20 minutes.