Beurre monté refers to melted butter that remains emulsified, even at temperatures higher than that at which butter usually breaks down. Beurre monté may refer either to the melted butter sauce itself, or to the method of making it.
Butter is an emulsion of about 2% milk solids, 80% milk fats (clarified butter), and about 18% water. At 70 °C (158 °F), butter normally breaks down into its components parts. But in a beurre monté, the butter is heated in such a way that the butter can stay emulsified even up to 82–88 °C (180–190 °F). It can then be used in many ways, including as a sauce, as an ingredient for other sauces, as a poaching medium, or as a resting medium for cooked meat.
In order to make a beurre monté, boil a very small quantity of water, i.e. 15-60 mL (1-4 tablespoons). Once water has come to a boil, turn the heat down and start whisking the cold butter into the water, one or two chunks at a time. Add more butter whenever the chunks have melted. Once the emulsion is started, more butter can be added at a time. Continue adding butter while whisking until one has the desired quantity of beurre monté. The beurre monté must then be held warm, but under 88 °C (190 °F) or else it will break.