Preheat the oven to 180°C (356°F).
Melt butter and dark chocolate together in a double boiler.
Add sugar and mix well.
Add eggs and beat thoroughly.
Sift cake flour and cocoa powder into the mixture and combine.
Pour the batter into the baking pan.
Bake in the oven at 180°C (356°F) for 20–25 minutes.