Caramelize 2 cups sugar by melting in heavy skillet. While sugar is caramelizing, in large saucepan combine the remaining 4 cups sugar, evaporated milk and syrup. Bring to boil. Pour caramelized sugar into saucepan mixture; stir constantly. Cook, stirring frequently, to 245 degrees or to firm-ball stage. (This is when a small amount of hot syrup dropped in chilled water will hold its shape.)

Remove from heat. Immediately add soda; stir vigorously as it foams up. Add margarine. Stir only until melted. Cool 20 minutes. Add vanilla. Beat until thick and creamy. Add chopped black walnuts. Pour into buttered 9-by-13-inch pan. Cool. Cut in squares. Keeps well in tight container.