Hollandaise sauce is the classic French emulsion of egg yolks, melted butter, and lemon juice — sometimes with a little vinegar or white wine reduction — whisked into a thick, pale-gold, buttery sauce. One of the five mother sauces of classical cuisine, it crowns eggs Benedict, asparagus, and poached fish with its rich, tangy silkiness. Making it is a rite of passage: the butter must be incorporated drop by drop into the warm yolks, with the temperature held in a narrow band — too cool and it refuses to emulsify, too hot and it scrambles. Because it is served warm and never boiled, hollandaise demands very fresh eggs and scrupulous hygiene; food-safety guidance often requires it be made to order and held briefly.