1. Line 8-or 9-inch square pan with foil.
2. Melt chocolate chips and 2 tablespoons butter with sweetened condensed milk, vanilla and salt in heavy saucepan over low heat. Remove from heat; stir in nuts. Spread evenly in prepared pan.
3. Melt marshmallows with remaining 2 tablespoons butter in medium saucepan over low heat. Spoon onto fudge. With table knife or metal spatula, swirl through fudge.
4. Refrigerate 2 hours or until firm. Remove fudge from pan; peel off foil. Cut into squares. Store loosely covered at room temperature. About 64 pieces or 2 pounds.
NOTE: For best results, do not double this recipe.