Heat oven to 350 F (325 F for glass baking dish). Microwave chocolate and margarine in large microwavable bowl on high 2 minutes or until margarine is melted. Stir until chocolate is completely melted. Stir sugar into chocolate until well blended. Mix in eggs and vanilla. Stir in flour and walnuts until well blended. Spread in greased 13x9-inch baking pan. Bake 30 to 35 minutes or until toothpick inserted, in center comes out with fudgy crumbs. Do not over bake. Cool in pan. Cut in squares.