In a saucepan beat the egg yolks with a wire whisk until they are thick and pale in color. Add the hot water, lemon juice and salt and pepper to taste, and beat vigorously.
Heat the butter just to bubbling in another saucepan. Pour it slowly into the egg mixture, beating rapidly with the whisk.
Place the sauce over very low heat and cook, stirring constantly, until sauce is properly thickened. Do not overcook or sauce will curdle. To restore a slightly curdled sauce, add two tablespoons cold light cream and beat quickly, off the heat.