First make the powdered brown sugar: Put the brown sugar (3/4 cup) in a blender and pulse for about 30-60 seconds until the sugar is powdered.

If using oat flour do the same with the oats. Store-bought oat flour can be ridiculously expensive, but it's super easy to make at home and so much cheaper. Just place 3/4 cup of rolled oats in a food processor and pulse them until they have acquired a powder-like consistency.

Combine all ingredients in a medium mixing bowl.

On a floured board or baking parchment, roll out the dough to about 0.4 inch (1 cm) thickness and cut it into star shapes using a cookie cutter.

Line a baking tray with parchment paper and place the star-shaped cookies on top.

Preheat the oven to 360 °F (about 180 °C) and bake for about 6-8 minutes. Make sure not to bake them too long, so they don't get hard. They're supposed to still be a bit soft.

In the meantime, make the icing: Combine all ingredients and stir until smooth. If it is too thin, add more powdered sugar. Let the cookies cool down and spread them evenly using a knife or the back of a spoon. Sprinkle them with some more cinnamon.