Place a 9x5-inch loaf pan in the freezer to chill.
To the bowl of a stand mixer, add the heavy cream. Start mixing at the lowest speed, and then gradually increase speed to medium until the cream starts to get thicker. (The gradual progression prevents you from spraying yourself with cream.)
The cream will thicken and start to peak. Mix on high speed until the cream is thick and stiff. This happens quickly, so watch it closely!
In a medium microwave-safe bowl, melt chocolate chips in 30-second increments, stirring in between until creamy. To the melted chocolate, add sweetened condensed milk, cocoa powder, vanilla, salt, and 1 cup of whipped cream.
Gently fold the chocolate and sweetened condensed milk mixture back into the mixing bowl with the rest of the whipped cream.
Add marshmallows and pecans (reserving some of each for topping), and gently fold them into the mixture.
Pour half of the mixture into the chilled loaf pan. Add 3 tablespoons of marshmallow fluff and swirl it in with a knife. Then, add the remaining ice cream mixture. Top with remaining fluff, and swirl again. Top with any remaining marshmallows and pecans and freeze for at least 5 hours, or until fully set up, before serving.