What thickens a hollandaise sauce?

Hollandaise sauce thickens primarily through the emulsification of egg yolks and fat, typically butter. When the egg yolks are gently heated and whisked, they create a stable emulsion with the melted butter, resulting in a creamy texture. The heat also helps to coagulate the proteins in the yolks, further contributing to the sauce's thickening. To achieve the right consistency, it's important to control the temperature and add the butter gradually.