In a medium saucepan, whisk together 1 ½ cups heavy cream, flour, and salt. Cook over medium heat until the mixture simmers. Stir frequently. In a medium bowl, lightly whisk the egg yolks. Gradually add the hot cream mixture to the egg yolks, whisking continuously. Return the mixture to the saucepan. Cook over medium heat until mixture thickens, stirring frequently. Remove from heat, stir in the vanilla and marshmallow fluff until fully blended. Pour the mixture into a shallow dish, cover with plastic wrap, refrigerate for 2 hours.
In a small saucepan, combine the chocolate and remaining cream. Warm over low heat until the chocolate is completely melted. Remove from heat, stir in the corn syrup. Pour the chocolate mixture over the chilled custard. Use a butter knife or the back of a spoon to swirl the chocolate into the custard. Cover and refrigerate for an additional 2 hours.