In a large heavy saucepan over med. heat, combine marshmallow cream,sugar,evaporated milk,butter,and salt. Stir constantly and vigorously while bringing to a full boil, and cook 5 min. stirring constantly. Remove from heat and allow to cool until lukewarm. Stir in melted chocolate until smooth. Add 1/2 C. hot water and vanilla. Stir until smooth. Allow to cool before filling. Fudge mixture will be soft.