Preheat oven to 350ºF. In a medium saucepan melt margarine and chocolate over low heat. Remove from heat and beat in sugar, eggs, milk and vanilla until well combined. Stir in flour, Sweet’N Low, baking soda and salt. Fold in nuts. Spread batter into an 8x8x2-inch baking pan sprayed with nonstick vegetable cooking spray. Bake 20 minutes or until a toothpick inserted in the center comes out clean. Cool on a wire rack before cutting into squares.