In a large mixing bowl, combine the all-purpose flour, ground almonds, ground pistachios, sugar, vanilla sugar, and a pinch of salt. Stir until well mixed.

Add the diced cold butter and the egg to the dry mixture. Knead until a smooth dough forms.

Wrap the dough tightly in cling film and chill in the refrigerator for about 30 minutes.

Preheat your oven to 175 degrees C (350 degrees F).

Roll out the chilled dough on a lightly floured surface to about 1 cm thick.

Cut out star shapes using a cookie cutter and place them on a lined baking sheet.

If using the optional filling, sprinkle half of the star shapes with ground pistachios and sugar, then top with a second star shape.

Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden brown.

Let them cool completely on a wire rack.

Mix the powdered sugar with cinnamon, adding lemon juice or milk gradually until you achieve a thick, spreadable consistency.

Once the cookies are cooled, cover them with the cinnamon glaze and decorate with small pistachio stars or chopped pistachios while the glaze is still wet.

Optionally, sprinkle with edible glitter or powdered sugar to make them sparkle.