In a medium, heavy saucepan cook chocolate and margarine or butter over low heat until melted, stirring frequently. Add milk, sugar, and corn syrup. Bring to a gentle boil over medium heat. Boil gently, stirring frequently, for 8 minutes or until mixture is thickened. Remove from heat. Cool slightly. If desired, stir in liqueur. Serve warm. To store, cover and refrigerate for up to 1 week. Makes 1 cup.