Name the essential ingredient in a "Hollandaise" sauce.

In a Hollandaise sauce, the essential ingredient is egg yolks. This sauce is classified as an emulsion, which is created by whisking egg yolks with an acid, typically lemon juice or vinegar, and then gradually incorporating melted butter to achieve a rich and creamy texture. The egg yolks serve as an emulsifying agent, allowing the fat from the butter to blend smoothly with the acid and produce a stable sauce.

While ingredients like butter and lemon juice are important in creating the flavor and consistency of Hollandaise, the egg yolks are fundamental to its structure and success. They provide the necessary proteins that stabilize the emulsion, making them indispensable to the sauce. Sour cream and milk are not components of traditional Hollandaise, which is why they do not serve as essential ingredients in this classic sauce.