In top of double boiler over simmering water, combine yolks with icing sugar. Using electric mixer, beat until foamy, about 1 minute. Gradually pour in wine, beating for about 5 minutes longer or until sauce doubles in volume and thickens; transfer to bowl. Stir in orange rind and liqueur if using. Beat egg whites until stiff glossy peaks form. Stir about one quarter into sauce; fold in remainder. Makes about 4 cups sauce. Formatted by Carole Walberg