Pour heavy cream into the bowl of a stand mixer fitted with a whisk attachment. Beat until stiff peaks form. Fold in sweetened condensed milk, 1 cup marshmallows, and hot cocoa mix. Transfer to a 9x5-inch loaf pan; smooth the top with a spatula. Top cream mixture with the remaining marshmallows.
Cover pan with plastic wrap and freeze until firm, about 5 hours. Remove from freeze to let soften, about 10 minutes.