Little is more frustrating in the kitchen than painstakingly beating a butter sauce into a perfect, smooth, thick consistency, then watching it break and crumble into grainy clumps. You may be inclined to throw the sauce into the trash in disgust, but don't despair quite yet. Chances are you can rescue the broken sauce and make it as good as new, if not better. The process is surprisingly simple and takes only a moment --- much less time than throwing away the broken sauce and starting anew.
Put 1 tbsp. of cold water into a metal bowl. Separate an egg. Discard or set aside the shells and white, pouring only the yolk into the bowl with the water.
Whisk the water and egg yolk together thoroughly with a metal whisk until they are completely blended.
Whisk the blended water and egg yolk continuously as you dribble a small amount of the broken sauce into the bowl. Keep whisking until the broken sauce and your egg mixture blend together completely. Then dribble in a little bit more of the broken sauce while still whisking.
Add more and more of the broken sauce, a little at a time, to your metal bowl. Do not stop whisking at any point while you do this. Your egg mixture will rescue the broken sauce, causing it to become whole and usable again.
If your finished sauce is too thick, whisk in a little bit of water to help thin it out. Because this process basically makes your sauce thicken all over again, it may be significantly thicker than it was before it broke.