Preheat the oven to 180°C.
Melt dark chocolate and butter using a double boiler.
Combine eggs and sugar in a bowl and mix.
Add the melted chocolate mixture to the egg mixture and blend.
Sift cake flour and fold it in with a spatula.
Pour the batter into a pan lined with parchment paper.
Bake in the oven at 180°C for 20-25 minutes.
Cool and cut into desired sizes.