Put egg yolks in the top of a double boiler. Beat with a wire whisk until smooth but not fluffy. Add lemon juice or mild vinegar, butter or margarine, hot water, salt and cayenne pepper. Set over very low heat in the double boiler. Beat until the sauce begins to thicken (about 5 minutes). The sauce will thicken more as it cools. Don't heat up to a boil if it cools before serving.