Preheat your oven to 350°F and line two standard muffin tins with 22 paper cupcake liners. Set aside while you prepare the batter. In a large mixing bowl, use an electric mixer to combine the chocolate fudge cake mix, milk, oil, eggs, dry hot cocoa mix, and dry chocolate pudding mix. Beat on medium speed for about 2 minutes, or until the batter is smooth and fully blended. Divide the batter evenly among the prepared cupcake liners, filling each about three-quarters full. Bake in the preheated oven for 15 to 18 minutes, or until a toothpick inserted into the center comes out clean or with just a few moist crumbs. Allow the cupcakes to cool completely before frosting.

To make the marshmallow buttercream, add the softened butter, marshmallow fluff, vanilla extract, and a pinch of salt to the bowl of a stand mixer. Beat together on medium speed until smooth and well combined, about 1 minute. Gradually add the powdered sugar, one cup at a time, alternating with small additions of heavy cream. Continue beating until the frosting becomes light, fluffy, and easy to pipe or spread. Once the cupcakes have cooled, frost them as desired and garnish each with a sprinkle of mini marshmallow bits and a drizzle of chocolate syrup.