Preheat oven to 350 degrees F. Line 2 muffin tins with about 18 paper liners and set aside.
In a large bowl, prepare the cake mix according to package directions or until smooth and blended, about 2 minutes. Portion the batter evenly among each muffin tin, filling about 3/4" of the way full. Bake for approx. 15-18 minutes or until a toothpick inserted near the center comes out clean or with light, moist crumbs. Cool the cupcakes completely.
Once cupcakes are cooled entirely, core them by using a small, sharp paring knife, careful not to cut the cupcake all the way to the bottom. Discard (or snack on!) the cores of the cupcakes. Set aside. In a medium bowl, combine the marshmallow fluff with about 3/4 of the vanilla frosting; gently fold together to combine.
Place the marshmallow mixture into a plastic ziploc bag and seal out the air. Snip off a corner of the bag and pipe the marshmallow mixture into the cored cupcakes, filling just about to the top of the cupcake. Refrigerate the cupcakes for about 20 minutes to briefly set the filling.
Remove the lid and foil liner of the chocolate frosting and microwave for about 30-45 seconds or until nearly melted. Dunk each cupcake into the chocolate frosting, allowing excess to drip off but work quickly as to not lose your cupcake's filling! Invert the cupcakes back onto a cooling rack and allow the chocolate frosting to briefly set on top. Then, fill another piping bag with the remaining vanilla frosting and pipe squiggles onto each cupcake for the signature appearance. Serve immediately and store leftovers airtight at room temperature.