01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - In a mixing bowl, whisk together the flour, cocoa powder, sugar, baking soda, baking powder, and salt until evenly distributed.
03 - Add the buttermilk, vegetable oil, eggs, and vanilla extract to the dry ingredients. Mix until just combined, then pour in the hot water and stir until the batter is smooth. The batter will be thin — this is normal.
04 - Divide the batter evenly among the prepared liners, filling each about two-thirds full.
05 - Bake for 18 to 20 minutes, or until a toothpick inserted into the center of a cupcake comes out clean. Transfer to a wire rack and let cool completely.
06 - Beat the softened butter with an electric mixer until light and creamy. Add the powdered sugar and beat for 1 minute. Mix in the marshmallow creme and vanilla extract, beating until fluffy. If the filling is too stiff, add milk one teaspoon at a time until it reaches a pipeable consistency.
07 - Using a small knife or cupcake corer, remove a small portion from the center of each cooled cupcake, saving the removed tops. Fill each cavity with cream filling using a piping bag, then replace the tops over the filling.
08 - Heat the heavy cream in a small saucepan until steaming but not boiling. Pour the hot cream over the chocolate chips and let sit for 2 minutes, then stir until smooth and glossy.
09 - Spoon or dip the tops of the cooled cupcakes into the ganache, coating evenly. Set aside until the glaze sets.
10 - Stir together the powdered sugar and milk until smooth and pipeable, adding more milk as needed. Transfer to a piping bag fitted with a fine tip or a zip-top bag with a tiny corner snipped off. Pipe the classic curly swirl across the top of each glazed cupcake.
11 - Allow the ganache and icing to fully set before serving. Enjoy at room temperature for the best texture and flavor.