Preheat the oven to 350 degrees F. Line muffin pans with 20-24 cupcake liners.
In a bowl mix the cake mix, egg, milk and butter together. evenly divide the mix into the cupcake liners. Bake at 350 degrees for about 15-18 minutes or until a toothpick inserted in the center comes out clean. Let the cupcakes cool completely, approximately 30 minutes.
In a bowl add the butter and blend on medium speed for 1 minute. Add the powdered sugar, and beat until creamy. Add the marshmallow cream and the vanilla and blend on low speed until combined. Place 2 oz of filling into a small pastry or zip lock bag fitted with a small lettering tip (Wilton # ), then the remaining filling (about 10 oz) into a second pastry bag fitted with a star tip.
To fill the cupcakes insert the star tip into the center of each cupcake and squeeze the pastry bag gently, making the cupcake begin to puff slightly. Start to lift the filling bag and stop squeezing before* you reach the top for a level surface. Repeat for the remaining cupcakes.
For The Ganache
In a small saucepan, heat the heavy cream on medium-low heat, swirling until the cream starts to steam, about 2-3 minutes. Do not allow this to boil!
Place the chocolate chips in a heat-resistant mixing bowl. Pour the steaming, warmed cream over the chopped chocolate and immediately cover the bowl with plastic wrap. Allow to sit undisturbed for 5 minutes. Remove the plastic wrap and gently whisk the melted chocolate and cream together until completely blended. If it seems too thin, let it cool for a bit it will thicken up.
While the ganache is still slightly warm and pliable, dip each cupcake into the ganache, allowing any excess to drip off.
Decorate the dipped tops with the remaining marshmallow filling by piping swirls on across the top of each cupcake. Using gentle but enough pressure starting at the far side of the cupcake, press the filling and allow it to swirl making e’s overlapping 6 to 7 times. Stop squeezing towards the end and pull the bag up to end the swirl.
It doesn’t have to be perfect! Hostess has machines that decorate their cupcakes.
Refrigerate the finished cupcakes uncovered for a minimum of 30 minutes to allow the ganache to set.
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Notes
These cupcakes are best chilled after serving, and can be enjoyed for up to 2 days after baking.
It is best to use a slightly larger star #21 tip to fill the cupcakes and a smaller #3 or Bismark writing tip for the swirls. I've linked them above in the tools. To ensure there is enough for both, separating the filling in two bags- about 2 oz in one for the swirls and the remaining filling in a second bag for filling will cover all the cupcakes.