Preheat the oven to 180°C.
Melt the butter and dark chocolate using a double boiler.
Add eggs and sugar to a bowl and mix well.
Pour the melted butter and chocolate into the egg mixture and combine.
Sift cake flour and cocoa powder and add to the mixture, stirring evenly.
Pour the batter into a pan lined with parchment paper.
Bake in the oven for 25 minutes.