Heat oven to 350°F. Grease and flour 13x9-inch pan. In large saucepan, melt 1/2 cup margarine and 1 oz. unsweetened chocolate over low heat, stirring until smooth. Remove from heat. Lightly spoon flour into measuring cup; level off. Stir in 1 cup flour and all remaining base ingredients; mix well. Spread in greased and floured pan.
In small bowl, combine 6 oz. of the cream cheese, 1/4 cup margarine, 1/2 cup sugar, 2 tablespoons flour, 1/2 teaspoon vanilla and 1 egg; beat 1 minute at medium speed until smooth and fluffy. Stir in 1/4 cup nuts. Spread over chocolate mixture; sprinkle evenly with chocolate chips.
Bake at 350°F. for 25 to 35 minutes or until toothpick inserted in center comes out clean. Remove from oven; immediately sprinkle with marshmallows. Return to oven; bake an additional 2 minutes.
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.