Beurre noisette

Beurre noisette (, literally: hazelnut butter, loosely: brown butter) is a type of warm sauce used in French cuisine. It can accompany savoury foods, such as winter vegetables, pasta, fish, omelettes, and chicken. It has also become a popular ingredient in contemporary American Italian Cuisine or even the classic American Chocolate chip cookie. It is widely used in making French pastry. It is notable for its deep yellow, almost brown, colour and having a nutty scent and flavour from the heating process.
Preparation.
When unsalted butter is melted over low heat and allowed to separate into butterfat and milk solids, the latter naturally sink to the bottom of the pan and, if left warming over gentle heat, will begin to cook slightly and turn a deep brown color. As they reach a toasty hazelnut colour, the pan is removed from the heat. The result is called beurre noisette, or brown butter. 
"Beurre noisette" may be used in its liquid state as a flavorful addition to many foods or cooled into a solid form. It has a nutty flavour and is particularly included in the batters for "madeleines" and "financiers".
If "beurre noisette" is not mixed after preparation, but separated to the firm (protein) and liquid (fat) components, the latter is the type of clarified butter known as "ghee" in South Asia and "samna" in the Middle East countries.