01 - Preheat oven to 350°F. Line a standard 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. Stir until just combined, then pour in the hot water and mix until the batter is smooth. The batter will be noticeably 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. Blend in the marshmallow creme and vanilla extract, beating until the filling is fluffy and smooth. If the consistency is too stiff, add milk one teaspoon at a time until reachable.
07 - Using a small knife or cupcake corer, remove a small portion from the center of each cooled cupcake, reserving the removed tops. Transfer the cream filling to a piping bag and fill each cavity. Replace the reserved tops to cover the filling.
08 - Heat the heavy cream in a small saucepan until steaming but not boiling. Pour the hot cream over the chocolate chips in a 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 filled cupcakes, or dip each cupcake top-down into the ganache. Allow the glaze to set at room temperature.
10 - Stir together the powdered sugar and milk until a smooth, pipeable icing forms, adding more milk drop by drop if needed. Transfer to a piping bag fitted with a fine round tip or a zip-top bag with a tiny corner snipped off. Pipe the classic squiggly swirl across the top of each glazed cupcake.
11 - Serve the cupcakes at room temperature for the best texture and flavor. Enjoy with a glass of cold milk or a cup of robust coffee.