01 - Preheat the oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - In a large mixing bowl, whisk together the flour, cocoa powder, granulated sugar, baking soda, baking powder, and salt until evenly distributed.
03 - Add the buttermilk, vegetable oil, eggs, and vanilla extract to the dry mixture. Mix until just combined, then pour in the hot water and stir until the batter is smooth and uniform. The batter will be noticeably thin.
04 - Divide the batter evenly among the prepared liners, filling each cavity 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 cylinder from the center of each cooled cupcake, reserving the tops if desired. Transfer the cream filling to a piping bag and fill each cavity. Replace the reserved tops if using.
08 - Heat the heavy cream in a small saucepan until steaming but not boiling. Pour the hot cream over the chocolate chips in a heatproof bowl and let sit for 2 minutes. Stir gently until the ganache is smooth and glossy.
09 - Spoon the ganache over the tops of the cooled cupcakes or dip each cupcake top directly into the ganache, allowing the excess to drip off. Set aside until the glaze is set.
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 a signature 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.