Preheat the oven to 180°C (356°F).
Melt the dark chocolate and butter together using a double boiler.
In a bowl, add sugar and eggs and mix well.
Add the melted chocolate and butter to the bowl and combine.
Sift the cake flour and cocoa powder into the mixture and mix.
Pour the batter into a pan and spread evenly.
Bake in the oven for 25 minutes.
Let it cool, then cut into pieces.