1) French sauce 
Beurre noir (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. Beurre noir is ...
Heating salted butter until dark brown and foamy but not smoking. A type of butter sauce called black butter sauce.
Black butter. Butter that has been cooked to a very dark deep brown; a sauce made with browned butter, vinegar, chopped parsley, and capers. It is usually served with fish.
a sauce of darkly browned butter, sometimes flavored with herbs, vinegar, etc.