Put the sugar and water in a saucepan and let boil five minutes. Let the syrup cool, then stir it slowly into the chocolate melted; add the vanilla. Let it stand in a pan of hot water until ready to serve; then add the cream or milk.
This sauce should be smooth and of the consistency of heavy cream. If it is to be used with ice-cream, omit the cream or milk and make it of the right consistency with water.