Mix flour, confectioners' sugar, butter, and egg in a stand mixer until it forms a firm dough.

Divide the dough into two equal parts.

Knead a tablespoon of cocoa powder into one portion of the dough.

Roll out the light dough into a rectangle and place it on a piece of parchment paper.

Roll out the dark dough and place it on top of the light dough.

Roll the two layers of dough together into a log and refrigerate for 1-2 hours.

Cut slices of about 3 millimeters and place them on a baking sheet lined with parchment paper.

Bake the cookies in a preheated oven at 200 degrees Celsius (392 degrees Fahrenheit) (conventional oven) for 8-10 minutes.

Let them cool on a wire rack.