Which ingredient serves as the primary fat source in a traditional hollandaise sauce?

In a traditional hollandaise sauce, butter serves as the primary fat source due to its rich flavor and smooth texture, which are essential characteristics of the sauce. Hollandaise is an emulsified sauce typically made by whisking egg yolks and lemon juice over a gentle heat while gradually incorporating melted butter. The emulsion of fat and liquid creates a creamy, luxurious sauce.

Butter not only contributes to the flavor profile but also provides the necessary consistency and stability for hollandaise. Its ability to emulsify is crucial, as the sauce requires a delicate balance of fat and protein to achieve the right texture. The choice of butter rather than other oils is important, as it imparts the traditional buttery taste that is iconic to hollandaise. Other oils such as olive oil or vegetable oil would not provide the same flavor or emulsification properties necessary for this classic sauce.