Here are a few reasons a sauce will break: Fat was added too quickly. Don't add the fat too quickly. Whisk cold butter into a sauce a little at a time to create an emulsification. Sauce got too hot. Barely heat the sauce so that the butter melts slowly and the liquid doesn't evaporate. Move it on and off the heat as necessary, while it is being incorporated. Sauce sat for too long. The emulsification of a sauce will only hold together for a limited time before a natural separation will occur. After a while, the emulsification will break, and the fat will separate from the sauce and float on top.