Melt the dark chocolate in the microwave, stirring every 30 seconds until melted. Set aside.
Using a stand mixer or handheld mixer, in a large bowl beat the heavy cream until stiff peaks form, set aside.
In a separate bowl using a mixer, beat together the sweetened condensed milk, vanilla, melted dark chocolate, cocoa powder, instant espresso powder, and kosher salt until smooth.
Add about ¼ of the whipped cream to the chocolate mixture and fold it in using a rubber spatula until fully combined. This step will take some effort but is necessary to make it easier to fold in the rest of the whipped cream.
Gently fold in the rest of the whipped cream until no streaks remain.
Pour ¾ of the chocolate ice cream mixture into a metal 8x8-inch baking pan and smooth it out. Set aside.
Slightly melt the marshmallow fluff until it becomes loose in the microwave, not liquid just enough to work with it easily. About 10-15 seconds.
Dollop the melted marshmallow fluff over the chocolate ice cream and use a knife to swirl it throughout.
Pour the remaining ¼ of the chocolate mixture on top, spreading it evenly. Cover and freeze for at least 6 hours or until firm.