Put the butter in a bowl. Add the icing sugar and mix until you get a uniform dough. Add the egg and mix again. Add the flour and mix again until the flour begins to disappear.
Shape it into a cylinder, wrap it in kitchen film and let it cool in the fridge for a couple of hours.
Sprinkle the countertop with a little flour and stretch the dough with a rolling pin. Cut the dough using molds or cortapastas with Christmas motives (firs, stars …). Cover the baking sheets with baking paper, spread the cookies on top and bake at 200ºC for 8-10 minutes.
For the white glaze mix:
Mix the egg white with the icing sugar, stir well until there is a homogeneous mass. Insert into a pastry bag and decorate the cookies hot. Decorate also with the other glasas. It serves.