While marshmallows are baking, in large saucepan, combine 1/4 cup margarine, milk, 1 oz. unsweetened chocolate and reserved 2 oz. cream cheese. Cook over low heat, stirring until well blended. Remove from heat; stir in powdered sugar and 1 teaspoon vanilla until smooth. Immediately pour frosting over puffed marshmallows and lightly swirl with knife to marble. Refrigerate 1 hour or until firm. Cut into bars. Store in refrigerator.