Sometimes a sauce will break no matter how attentive you are to its whims and needs. (See our post here with the most common reasons why emulsified sauces break.) It's frustrating and discouraging, especially if guests are at the table and waiting for your finishing touch.
But never fear! You can still pull off a fantastic sauce without missing a beat.
If your sauce is just starting to break, little droplets of fat will start to form around the edges of the bowl. When you see this, hold off on adding more fat for the moment and add a little liquid instead.
Use a teaspoon or two of whatever liquid you've used as a base (like water, wine, or vinegar) and whisk vigorously. The sauce should tighten up in a few seconds and the fat droplets will get suspended back into the emulsion. If the sauce isn't thick enough yet, you can pick back up with adding the fat one teaspoon at a time.
If your sauce has broken completely, the fat and liquid have separated and the sauce will look grainy and thin. To save this one, we'll need to take a few extra steps.
In a separate bowl, whisk together one egg yolk and tablespoon of whatever liquid you've been using as a base. Whisking constantly, add the broken sauce to this egg yolk one teaspoon at a time. This will form a fresh emulsion and new stable sauce.
If you're making a warm sauce and the eggs start to cook, unfortunately there's not a lot you can do to save it. If you're down to your last egg or stick of butter, you can strain out the curdled egg and begin a new sauce using a fresh egg and the old sauce like we describe above.
(Photo Credit: The American Egg Board)