In a medium size bowl, combine chips and marshmallow cream. In a small saucepan mix together sugar, butter and evaporated milk. Cook mixture over low heat and gradually bring to a boil. Boil for 5 minutes. Pour mixture over chocolate and marshmallow and stir in salt and vanilla. Stir until mixture melts together. Spread fudge in lightly buttered square baking dish. Let stand for several hours and then cut into squares.