Sublime Hot Fudge Sauce

In 2-quart saucepan, heat heavy or whipping cream, sugar, chocolate and corn syrup over medium heat until mixture comes to a boil, stirring occasionally. Cook 4 to 5 minutes longer, until sauce thickens slightly (mixture should be gently boiling), stirring constantly.

Remove saucepan from heat; stir in margarine or butter and vanilla until smooth and glossy. Serve immediately, or let cool completely, cover and refrigerate. (Do not cover sauce until it is cold, or the water from condensation will make it grainy.)