Monter au beurre

Monter au beurre, French for "to mount with butter", means whisking or swirling a few pieces of cold butter into a hot sauce just before serving. The butter melts into the sauce rather than floating on top, which makes it glossy, rich and a little thicker. It is the usual last step of a pan sauce.

How to do it

1. Finish and season your sauce, and reduce it to slightly thinner than you want it.
2. Take the pan off the heat, or turn it to very low.
3. Add cold butter cut in small cubes, about 1 to 2 tablespoons for every 120 ml (1/2 cup) of sauce.
4. Swirl the pan or whisk until each piece has melted before adding the next.
5. Serve at once.

How to tell it is done

The sauce looks shiny and opaque, coats a spoon lightly and has no pools of oil on the surface.

Why it works

Butter is an emulsion of fat in water, held together by milk proteins. When cold butter melts slowly into a sauce that is hot but not boiling, those proteins keep the fat spread in tiny drops, giving body and shine. Beurre blanc uses the same method with a lot more butter.

Common mistakes

• Boiling the sauce after the butter goes in. The emulsion splits and the sauce turns greasy.
• Using soft or melted butter, which melts too fast to emulsify.
• Adding it too early and then reducing further.
• Adding so much butter that it hides the other flavors.

In recipes

You may see "swirl in the butter", "finish with cold butter" or "mount the sauce with butter". All mean this method. Reheating a sauce finished this way is hard, so add the butter just before serving.