Whisking a knob or two of cold unsalted butter into a warm pan sauce or vegetable purée has long been a secret of many restaurant chefs as a quick way to create satiny-smooth sauces with an incomparable sheen and velvety texture. The technique, known as monter au beurre, is based on the same principle as making a beurre blanc—the butter must not melt so much that the butterfat separates, but rather it must soften to emulsify and thicken the sauce slightly.
• Whisk the cold butter, a little at a time, into a warm sauce over low heat. The butter should incorporate into the sauce without melting into oily puddles.
• For 1/2 cup pan sauce, add 1 to 2 Tbs. butter. Using less may cause the butter to separate out, more may obscure the flavor of the sauce. The proportions are less important with vegetable purées since the sauce will already be emulsified by the vegetable solids.
• Don’t let a sauce boil once you’ve finished it with butter.