Cream the butter and sugar in a standard mixer equipped with a paddle attachment, just until mixed. Don't fluff up the mixture by incorporating air. The mixture should be smooth, with no lumps of butter. Add the yolks, one at a time, and process to mix just until incorporated, scraping down the bowl once. Add the flour and mix until incorporated. Scrape the bowl and mix for a few more seconds. Wrap in plastic wrap and chill for one hour or up to 3 days. The dough can be frozen for up to 2 months. This recipe will make 2 9-inch tarts.