Sauce Beurre Blanc-White Butter Hollandaise Sauce

In a saucepan cook 3 shallots, finely diced, with 6 tablespoons of the liquid in which fish was cooked, 1/4 cup wine vinegar, salt, and pepper until the liquid is reduced to about 1/4cup. Partially remove the pan from the heat and add, bit by bit, ½ pound sweet butter, beating constantly with a piano wire whip until the butter is melted and the sauce is creamy white, thick, and foamy