Beurre Noir

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.
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(burr-nwahr) (French) Butter cooked to a dark brown, then adding capers and a dash of vinegar.