Pour the flour in a large bowl, together with the cold butter cut into cubes, 4 egg yolks, sugar, vanilla seeds, baking powder and add lemon (or orange) zest.
Mix everything into a single big block.
Place the dough on a board and keep kneading until even.
Roll out the dough with a rolling pin on a floured surface, so that it won't stick. Make sure it has about 3-4 mm thickness.
Use the star-shaped cutters or create stars manually.
Place them on baking sheets, then brush them with egg whites and sprinkle brown sugar on top and/or use coloring for icing.
Bake them in the oven at 180 degrees (356 F) for around 13-15 minutes. Make sure to look at them from time to time and let them in the oven until they have a beautiful, golden color.