The objective to making a good accompanying sauce is not to detract from the original flavor of the item in witch it is served. Generally, a sauce has four main purposes. Properly used, the unique qualities of roux will thicken, color and flavor the sauce thickened. A popular thickening agent. Add the mixture to rapidly boiling liquid and cook for 10 minutes.
Kneaded butter is a mixture of butter and flour in equal proportions.
Unlike roux, the kneaded butter must be cooked out in the sauce after being added. Egg yolks need to be tempered with a small amount of the sauce being thickened. This requires constant stirring. The benefits of using egg yolks as thickeners is the lustering sheen and rich flavor they create. Sauce consistancy is achieved by reducing the heavy cream.