Sift the flour, cocoa powder, baking powder, baking soda, and salt into a large bowl. Add the sugar and whisk to combine.
3. Step
Add the warm water, buttermilk, eggs, and vanilla and whisk until combined.
4. Step
Line 2 muffin tins with paper liners.
5. Step
Scoop the batter into prepared muffin tins, filling each cup a little more than halfway.
6. Step
Bake for about 20 minutes, until the tops of the cupcakes spring back when gently pressed. Let cool completely.
7. Step
Make the marshmallow filling: In a medium bowl, whip the heavy cream with an electric hand mixer until soft peaks form. Add the marshmallow cream and whip until stiff.
8. Step
Transfer the filling to a piping bag fitted with a medium round tip or a zip-top bag with a corner snipped off.
9. Step
Use the end of a whisk or another utensil to push down the center of each cupcake.
10. Step
Pipe the filling into the cupcakes, leveling off the tops if needed. Freeze for 15 minutes while you make the ganache and icing.
11. Step
Make the ganache: Add the chocolate chips and heavy cream to a medium microwave-safe bowl. Microwave in 30-second increments, until melted. Mix together until smooth.
12. Step
Make the icing: In a medium bowl, stir together the powdered sugar and milk until smooth. The icing should fall off the spatula in ribbons. If it is too thin, add a bit more powdered sugar.
13. Step
Transfer the icing to a piping bag fitted with a small tip or a zip-top bag with a corner snipped off.
14. Step
Dip the top of each cupcake in the ganache, then set aside to dry for 10 minutes.
15. Step
Pipe icing curls across the center of the cupcakes and let dry for 15 minutes at room temperature.
