Preheat oven to 325 degrees. Grease a 9-by-13-inch pan. Melt 1 cup margarine over low heat in flat-bottomed pan. Add 2 squares chocolate; stir until melted. Add granulated sugar; mix well. Stir in eggs. Remove from heat. Stir well. Add flour and nuts; mix just until blended. Pour into prepared pan; bake for 30 minutes. Just before crust is done baking, melt remaining 2 squares chocolate with remaining 1/4 cup margarine. Remove crust from oven; top immediately with marshmallows. (Marshmallows will melt from heat of crust.) Combine melted chocolate mixture with powdered sugar and milk; stir well. Pour chocolate over melted marshmallows; spread evenly. Let cool thoroughly before cutting into squares. Yield: 4 dozen bar cookies.