Hollandaise

Hollandaise... that bright, velvety concoction of egg yolks, butter, and lemon juice has a reputation for being difficult to make. In part, that's because the delicate proteins in egg and butter can unwind and lose their ability to emulsify if overheated, causing your sauce to break and become oily and thin. Even a sauce that comes out silky smooth and rich at first can quickly lose its appeal if allowed to cool too much. Timing, temperature, and proportion must be carefully controlled in order to yield hollandaise! 

Modernist technique goes a long way with this mother sauce: Sous vide cooking allows for convenient temperature control (and thus avoidance of overheating). Serving the sauce from a whipping siphon makes it easy to reserve until you're ready to serve it but also aerates the mixture, lifting and lightening it for a softer, more delicate texture.