Melt dark chocolate and butter together using a double boiler.
Add sugar to the melted chocolate and butter, and stir well.
Quickly mix in the eggs.
Sift cake flour and cocoa powder into the mixture and combine.
Fold in the chocolate chips.
Pour the batter into a baking pan.
Bake in a 180°C oven for 20–25 minutes.
Let cool, then cut into pieces.