Bring butter and eggs to room temperature.

Cream the butter in a bowl until smooth.

Add sugar or powdered sugar and mix at low speed.

Add egg yolks one at a time, mixing until creamy and light.

Sift the flour into the bowl and mix to form a dough.

Divide the dough into two portions. Leave one plain and mix cocoa powder into the other.

Wrap each dough portion in plastic wrap and refrigerate for 1 hour.

Roll out small portions of each dough into long strips.

Alternate strips of plain and cocoa dough, pressing them together gently.

Shape the combined dough into a rectangular or round log, wrap in plastic, and refrigerate for 30 minutes.

Preheat the oven to 170°C (340°F). Slice the dough log into 7-8mm thick pieces and place on a baking tray.