Finely chop the dark chocolate and melt it using a double boiler.
Mix butter and sugar and whip until creamy.
Add eggs one at a time, mixing thoroughly after each addition.
Add the melted dark chocolate and mix well.
Sift cake flour and cocoa powder, then add and gently fold with a spatula.
Pour the batter into a pan lined with parchment paper.
Bake in the oven at 180°C (356°F) for 20–25 minutes.
Let the bread cool before serving.