French for "hazelnut butter," beurre noisette (pronounced "bur nwah-zet") is butter that has been cooked until it has a golden brown color and a nutty flavor. Contrary to its name, this sauce, which has been used by French chefs since medieval times, does not actually contain hazelnuts. Before serving, it's often strained through a fine sieve to remove darkened milk solids, and acid (such as lemon) may be added to cut through some of its rich flavor.
In French cuisine, beurre noisette, which is known in English as browned butter, is typically used to add character to vegetable, fish, egg, and even brain dishes.

Source: Flickr User thibaut nazare