Cut the chocolate into small pieces and set aside. Combine the sugar and cream in a heavy 3-quart pan, stirring to dissolve. Bring to a boil, then cook for 6 minutes from the time it comes to a boil. Remove from the heat and add the chocolate and butter, stirring until melted. Add the vanilla. Refrigerate until well chilled, but not solid. Stir vigorously with a wooden spoon until it is a soft spreadable consistency.