Line a 8x8 or 9x9 inch square pan with parchment paper and set it aside.
Add the sugar, butter, and evaporated milk to a large heavy duty sauce pot over medium heat. 2 cups granulated sugar, 1/2 unsalted butter, 6 oz evaporated milk
Stir the mixture occasionally as it comes to boil. Once the mixture is at a full rolling boil, stir it constantly until the fudge reaches 234 F (this will take 4 to 5 minutes)
Remove the pot from the heat and add in the chocolate and marshmallow cream. Stir the mixture until the chocolate is melted and everything is stirred together. 12 oz chocolate chips, 5 oz marshmallow fluff
Add the vanilla extract and salt and stir it into the mixture. 1 tsp vanilla extract, 1/2 tsp salt
Pour the mixture into the prepared pan and spread the fudge to the corners, and spread the top evenly. Let it cool for at least an hour before adding the frosting on top.