In a large saucepan, bring sugar, milk and salt to a full boil for 5 minutes, stirring constantly. Remove from the heat. Place marshmallows, butterscotch chips and vanilla in a heat-resistant bowl; pour sugar mixture over, stirring until marshmallows are blended. Pour into a greased 9-in. square pan. Cool at room temperature. Melt chocolate chips in a double boiler or microwave; spread over fudge and immediately sprinkle with nuts. Cool before cutting into squares.