Mix butter, sugar, and flour well in heavy saucepan or top of double boiler. Add boiling water; cook over medium heat, stirring often* until sauce becomes thick and transparent. Cover and let simmer about 10 minutes. Stir twice. Whip cream until stiff, and fold into hot sauce. Keep covered until ready to serve. If it has cooled, warm slowly before serving. Please, no vanilla or other flavoring! This is a very tasty, buttery sauce.