Combine sugar, salt, milk and butter in a saucepan; mix well. Turn heat on high, stirring constantly until it boils; turn down to medium and boil 15 minutes. Scrape sides and stir every 2 minutes. When mixture reaches soft ball stage remove from heat. Add vanilla, marshmallows and chocolate chips. Mix well and turn into greased pan. Let stand 10 minutes, then cut into squares.