To SERVE four or more:
1/2 pound unsalted butter
3 egg yolks
1 tablespoon water
Salt and pepper

MELT THE butter in a saucepan and keep it lukewarm. In a very heavy saucepan (or in the top of a double boiler), beat together the egg yolks and water with a whisk. Season with salt and pepper.

Cook the yolks over a very low heat (or barely simmering water) until they begin to thicken noticeably. Now pour in bit by bit the lukewarm melted butter, stirring fast and constantly.

If the sauce thickens too much, thin it cautiously with small spoonfuls of cold water. Taste again for seasoning, and keep the sauce over warm water until you are ready to serve it.