In a medium saucepan, heat and stir chocolate pieces, condensed milk, butter, vanilla, and salt over medium heat until melted and smooth. Remove from heat; stir in marshmallows just until combined. Spread mixture evenly in the prepared pan. Using a table knife or thin metal spatula, swirl marshmallows through fudge in pan until marshmallows are mostly melted. Cover and chill about 2 hours or until firm.