Line a 9" square pan with foil extends over sides of pan; butter foil. In a saucepan, combine chocolate chips, sweetened condensed milk, two tablespoons margarine, vanilla extract, and salt. Cook until melted and smooth. Remove from heat and stir in walnuts. Spread evenly into prepared pan. In a smaller sauce pan, combine marshmallows and remaining margarine. Cook until melted and smooth. Spread on top of fudge. Cut through both layers with a knife several times for the swirled effect. Chill for two hours, or until firm. Turn fudge onto cutting board. Peel off foil and cut into squares.