In a heavy saucepan melt the chocolate with the butter and the corn syrup over moderately low heat, stirring, add the cream and the sugar, and cook the mixture, stirring, until the sugar is dissolved. Bring the mixture to a boil over moderate heat and boil it, without stirring, for 8 minutes. Remove the pan from the heat and stir in the vanilla and the salt. Serve the sauce hot over ice cream. The sauce keeps, covered and chilled, for 1 month. (Let the sauce cool completely before covering it; any condensation will make it grainy. Reheat the sauce, uncovered, in a double boiler.)

Makes about 1 1/3 cups.