Butter

Butter has long been used in classic French cooking to finish sauces thickened with flour and for certain pan-deglazed sauces. When nouvelle cuisine was at its peak, it became popular as a liaison for flourless sauces and in fact became the thickener of choice for made-to-order brown sauces. Some such sauces were (and are) made almost entirely with butter.

When butter is whisked into a hot liquid, it forms an emulsion, similar to the action of egg yolks. The milk solids and proteins contained in the butter act as emulsifiers that keep microscopic globules of fat in suspension and give butter sauces their characteristic sheen and consistency. Because the milk solids contained in the butter are what maintain the emulsion, sauces and cooking liquids cannot be thickened with clarified butter unless another emulsifier is used. In fact, cold butter, itself an emulsion, is preferable to warm butter, which may have begun to turn oily.