For the spiral cookies, beat butter and sugar in a bowl until fluffy. Mix starch and flour and mix with the butter mixture.
Add the egg yolk and milk and mix everything into a firm dough. Divide the dough in half and add the cocoa to one half. Wrap both halves of the dough with plastic wrap and let rest in the icebox for 1 hour.
Roll out the dough into 3 mm thick sheets and place them on top of each other. Roll up lengthwise and wrap in plastic wrap. Let rest in the icebox for another 30 minutes.
Preheat oven to 180 °C / hot air. Cut the dough roll into slices about 1 cm thick and place them on a baking sheet. Bake the spiral cookies in the oven for 10-15 minutes. Allow to cool and serve.