By Harold McGee
Perhaps the simplest of sauces is the pat of butter dropped on a heap of hot vegetables, or stirred into rice or noodles, or drawn across the surface of an omelet or steak to give a sheen. Melted butter can be enlivened with lemon juice, or “clarified” to remove the milk solids (see below). Beurre noisette and beurre noir, “hazel” and “black” butter, are melted butter sauces that the French have used since medieval times to enrich fish, brains, and vegetables. Their flavor is deepened by heating the butter to about 250°F/120°C until its water boils off and the molecules in the white residue, milk sugar and protein, react with each other to form brown pigments and new aromas (the browning reaction). Hazel butter is cooked until it’s golden brown, black butter until it’s dark brown (truly black butter is acrid). They’re often balanced with vinegar or lemon juice, which should be added only after the butter has cooled below the boiling point; otherwise the cold liquid will cause spattering and the lemon solids may brown. On their own, they lend a rich nutty flavor to baked goods.