Melt dark chocolate and butter using a double boiler.
Add sugar and eggs to a bowl and mix.
Pour the melted chocolate and butter into the bowl and combine.
Sift cake flour and cocoa powder, add to the mixture, and gently fold together.
Pour the batter into the pan and spread evenly.
Bake in a preheated oven at 170°C (338°F) for 20~25 minutes.