Preheat oven to 350 degrees F. Line 2 muffin tins with 22 paper liners. Set aside.
In a large bowl, beat together the cake mix, milk, oil, eggs, hot cocoa packet, and dry pudding mix together with an electric mixer until combined and smooth, about 2 minutes. Portion the batter evenly among the muffin cups, filling about 3/4 full.
Bake for approximately 15-18 minutes or until a toothpick inserted near the center comes out clean or with moist crumbs. Cool completely.
To make the frosting: Cream together the butter, marshmallow fluff, vanilla and salt together in a stand mixer until combined and smooth, about 1 minute. Gradually add in the powdered sugar, about one cup at a time, alternating with the heavy cream until frosting is light and fluffy.
Pipe or spread the frosting onto the cooled cupcakes. Garnish with marshmallow bits and the chocolate syrup.