Melt dark chocolate and unsalted butter together using a double boiler.
Combine sugar and eggs in a bowl and mix well.
Add the melted chocolate mixture to the egg mixture and stir until well combined.
Sift cake flour and cocoa powder into the mixture and fold quickly.
Pour the batter into a baking pan and spread evenly.
Bake in a preheated oven at 170°C (338°F) for 25 minutes.