Beurre noisette (French: literally, "hazelnut butter" - though the hazelnut reference applies only to the colour - , sometimes loosely translated as "brown butter") is frequently used in French pastry production. It can also be used as a warm sauce to accompany a variety of savoury foods such as winter vegetables, pasta, fish, omelettes, chicken, etc.
Unsalted butter is melted over low heat and allowed to separate into butterfat and milk solids. The milk solids naturally sink to the bottom of the pan and, if left over gentle heat, will begin to brown. As the milk solids reach a toasty hazelnut colour, the pan is removed from the heat. Beurre noisette may be used in its liquid state, or cooled to a solid form. It has a characteristic warm, nutty flavour, and is particularly included in the batters for madeleines and financiers.
Here is our recipe for making beurre noisette in a Thermomix