Brownies: Grease and flour a 13x9 in. Pan. Melt 1/2 c. Butter and one square chocolate. Add 1 c. Sugar, 1 c. Flour, 1/2 c. Nuts, 1 t. Baking powder, 1 t. Vanilla and 2 eggs. Mix well and spread in pan. Filling: Combine 6 oz. Cream cheese, 1/2 c. Sugar, 2 T. flour, 1/4 c. Softened butter, 1 egg and 1/2 t. Vanilla. Beat one minute on medium until smooth and fluffy. Stir in 1/4c. Nuts. Spread over chocolate mixture. Sprinkle with chocolate bits. Bake in preheated 350 degree oven 25 to 35 min. (use toothpick test). Remove from oven, sprinkle with marshmallows, and bake 2 more minutes. Topping: Melt 1/4 c. Butter, 1 square chocolate, 2 oz. Cream cheese and milk. Stir in powdered sugar and 1 t. Vanilla until smooth. Pour over bars and swirl with a spoon. Cool and cut into bars. Store in refrigerator.