Beurre noisette is created when unsalted butter is gently melted over a low heat to separate the milk solids and the butter fats.
The milk solids sink to the bottom and as they continue to cook begin to brown, until they reach a hazel colour at which point the butter is removed from the heat.
Beurre noisette can be used hot as a liquid with fish such as lemon sole, or have sage added to dress pasta dishes. When cold it congeals into a solid. If however the liquid fat is kept separate from the solids this is ghee, a type of clarified butter, used in indian and other asian cookery.
Beurre noisette, with its nutty flavour, is used by pastry chefs in the preparation of such biscuits as madeleines and financiers, and cakes such as genoise sponge.