Melt dark chocolate and butter over a double boiler.
Add sugar to the melted chocolate and mix well.
Add eggs at room temperature one by one and mix.
Sift and add cake flour and cocoa powder, then combine.
Add nuts to complete the batter.
Pour the batter into a pan lined with parchment paper.
Bake in a preheated oven at 180°C for 20–25 minutes.