Beurre noir

Beurre noir 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. It is typically served with eggs, fish, or certain types of vegetables.

Source: Wikipedia