Egg yolks make wonderful thickeners--imparting both a rich flavor and velvety smooth texture--but they're tricky to use. You can't just whisk them into a simmering sauce--they'd curdle on contact. Instead, you need to "temper" them by adding some of the hot liquid to the egg yolks, whisking the mixture together, and then adding it to the sauce. To prevent the yolks from coagulating, you need to keep the sauce below 190°, although this rule can be broken if the sauce has a lot of flour in it. Finally, never cook sauces with egg yolks in aluminum pans or they'll turn gray.