In French, the term ‘beurre noisette’ loosely translates to ‘brown butter’ (literally, it’s hazelnut butter). It is unsalted butter that has been gently heated until the milk solids separate from the butterfat, and the milk solids reach a hazelnut colour. This browning enriches the butter with a nutty flavour.
Butter will begin to brown at 266 °F (130 °C).