Butter sauces need to be kept warm to prevent the butter from solidifying, and are best held at around 145°F/63°C to discourage the growth of bacteria. Because the egg proteins slowly continue to bond to each other at this temperature, the cook should stir the sauce occasionally to keep it from thickening. The container should be covered to prevent the sauce’s moisture from evaporating and overcrowding the fat droplets, and to prevent the formation of a protein skin on the surface.
Curdled egg sauces can be rescued by straining out the solid bits of protein, keeping the whole mess warm, beginning with another warm egg yolk and one tablespoon/15 ml water, and slowly whisking the sauce into the new yolk. The same technique will revive a sauce that has been refrigerated and so had its butterfat crystallized; the crystals melt to form fat puddles when the sauce is simply rewarmed.