4 tablespoons white wine vinegar
2 tablespoons dried tarragon
1/4 cup finely chopped shallots or scallions
1/2 teaspoon salt
Freshly-ground pepper
3 tablespoons water
3 egg yolks
1/2 pound butter

In a heavy nonreactive saucepan place the vinegar, tarragon, shallots, salt and pepper. Bring to a boil and cook down until moisture almost disappears. Add water and egg yolks and stir briskly with a wire whisk over low heat until the sauce has thickened to the consistency of mayonnaise. In another saucepan, melt butter. Cool butter to the touch and slowly, in a thin stream, add to the yolk mixture. Sauce should be warm, not hot.