Stir the fat with the sugar until thick and creamy.
Finely grind the oat flakes in a coffee grinder or meat grinder and gradually add to the sugar mixture with the flour, baking powder, cinnamon and milk (about an eighth of a liter).
Divide the dough. Mix one part with cocoa powder if you want the cookies dark colored.
Use a syringe to pipe the dough in rings, horseshoes or possibly S-shapes onto the greased baking sheet.
Bake for about 10 minutes.