The secret of exquisitely smooth hollandaise sauce is forming a perfect emulsion. Egg yolks are naturally emulsified, and when melted butter is whipped or whirled into them, the fat and liquid won’t separate if the eggs aren’t overcooked. The trick is to get the mixture to a bacteria-safe temperature without cooking the yolks enough to solidify them and make the sauce separate. If this happens, you can save it by putting 2 tablespoons of water in a blender or another bowl and whirling or whisking in the broken sauce.