In a large heavy saucepan over medium heat, combine marshmallow cream, sugar, evaporated milk, butter and salt. Stir constantly and vigorously white bringing to a full boil and cook for 5 minutes, stirring constantly. Remove from heat and allow to cool until "just" warm. Stir in melted chocolate until smooth. Add 1/2 cup hot water and vanilla. Stir until smooth. Cool until able to pipe into cupcake cavity. Mixture will be soft. Fill a pastry bag fitted with a #2A tip (or #230 bismarck tip for mini cupcakes) with the fudge filling. Recipe and image taken with permission from Vi Whittington and Autumn Carpenter's Book, Creative Cupcakes.