Start by mixing the dry ingredients in a bowl; flour, sugar, and ammonium bicarbonate.
Cut the butter into small pieces and add it to the mixture.
Knead the dough gently and divide it into 2 pieces.
Shape them into rolls approximately 6 cm in diameter and cover them with a cloth. Place them in the freezer for 40 minutes or in the refrigerator for a couple of hours.
Mix cinnamon and sugar in a small bowl, and beat the egg in another small bowl.
Take the rolls from fridge/freezer and cut them into thin slices. Brush with egg and sprinkle cinnamon sugar onto the cookie.
Carefully transfer them to a baking sheet lined with parchment paper.
Place the baking sheet on one of the upper racks in the oven and bake them at 200 degrees Celsius (392 degrees Fahrenheit) in a conventional oven.
The baking time ranges from 5-8 minutes, depending on your oven.
Let them cool afterwards on a cooling rack.