French for black butter, the butter is cooked over a low heat until it turns dark brown, not black. It is usually flavored with vinegar or lemon juice, capers and parsley and served as a sauce with eggs, fish, sometimes, vegetables. One of the most famous classic dishes is calves brains in black butter.
Beurre noir (French: black butter) is melted butter that is cooked over low heat until the milk solids turn a very dark brown. As soon as this happens acid is carefully added to the hot butter, usually lemon juice or a type of vinegar. Some recipes also add a sprig of parsley, which is removed from the hot butter before the acid is added.