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.