In a 2-quart saucepan combine margarine, sugar, cocoa, egg and vanilla. Cook over medium heat, stirring constantly until margarine is melted and mixture is smooth, 1 to 2 minutes. Stir in remaining crust ingredients. Press into a 9 x 13-inch pan and chill 15 minutes. In a small mixer bowl combine filling ingredients. Beat at medium speed, scraping bowl often until smooth, 2 to 3 minutes. Spread evenly over crust. Chill 15 minutes. In a 1-quart saucepan combine chocolate pieces and milk. Cook over low heat, stirring occasionally until chocolate is melted, 2 to 3 minutes. Spread evenly over bars. Cover; chill 2 to 3 hours or until firm. Cut into bars. Store, covered, in the refrigerator. Yield: 4 dozen.