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Heat oven to 350 F. Grease 13×9 inch pan. In microwave bowl, melt ½ cup butter and 1 oz. unsweetened chocolate 30-60 seconds, stirring until smooth.
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Spoon flour into measuring cup. Add 1 cup flour and remaining base ingredients; mix well. Spread into greased dish.
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In small bowl, combine all filling ingredients except chocolate chips. Beat 1 minute at medium speed until smooth and fluffy. Spread over chocolate mixture; sprinkle evenly with chocolate chips.
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Bake at a 350* F for 25 to 35 minutes until toothpick inserted in center comes out clean. Immediately sprinkle marshmallows over top. Return to oven and bake an additional 2 minutes.
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Meanwhile, in another glass dish, combine ¼ cup margarine, milk, 1 oz. unsweetened chocolate and reserved 2 oz. cream cheese; heat and stir until well blended. Stir in powdered sugar and 1 teaspoon vanilla until smooth. Immediately pour frosting over marshmallows and lightly swirl with knife to marble. Refrigerate until firm; cut into bars.