Combine the butter and sugar in a medium mixing bowl or the bowl of an electric mixer fitted with paddle and beat on medium speed for 5 minutes until the mixture is fluffy, soft and almost white in color.
Beat in the vanilla and the egg yolk and continue beating for another two minutes until the mixture is soft and resembles butter cream.
Place the flour in a strainer and sift it all at once over the butter mixture using a rubber spatula.
Fold the dough until no traces of flour remain.
Scrape the dough onto the center of a 12 inch square of plastic wrap and shape it into a rough disk.
Cover with another 12 inch piece of plastic and press the dough until it is about 1/4 inch thick.