Preheat the oven to 170°C.
Melt dark chocolate and butter using a double boiler.
Whisk together eggs and sugar until frothy.
Add the egg mixture to the melted chocolate and mix well.
Sift cake flour and cocoa powder into the mixture and combine evenly.
Pour the batter into a baking pan and smooth the surface.
Bake in the preheated oven for 25 minutes.
Cool on a wire rack.