Preheat the oven 350 °F. Line three baking sheets with parchment paper.
Cream the butter and sugar in a bowl of a stand mixer fitted with a paddle attachment on medium-low speed until light and fluffy (about 3 minutes). Add the egg yolks one at a time mixing well after each addition.
Scrape down the sides of the bowl with a rubber spatula, add the vanilla, salt and orange zest (if using), mix for another minute or so until combined. Add the flour and mix on low speed until fully incorporated (about 2-3 minutes).
Transfer the dough to a lightly floured work surface and roll it out to about ¼ inch thickness. Using 2-3 inch cookie cutters cut the dough into the shapes of your choice. Transfer the unbaked cookies to parchment covered cookie sheets spacing them about 2 inches apart. Decorate with sprinkles or caster sugar (if using) and bake for about 15-20 minutes until lightly golden. Allow the cookies to cool for about 5 minutes then, using a large spatula, transfer them to a cooling rack. Cool completely before enjoying.
