01 - Preheat oven to 350°F. Line a standard 12-cup muffin tin with paper liners and set aside.
02 - In a large mixing bowl, whisk together the all-purpose flour, unsweetened cocoa powder, granulated sugar, baking soda, baking powder, and salt until evenly distributed.
03 - Add the room-temperature buttermilk, vegetable oil, eggs, and vanilla extract to the dry mixture. Beat until just combined, then pour in the hot water and stir until the batter is smooth and lump-free. The batter will appear quite thin — this is normal.
04 - Divide the batter evenly among the prepared liners, filling each about two-thirds full to allow room for rising.
05 - Bake for 18 to 20 minutes, or until a toothpick inserted into the center of a cupcake comes out clean. Transfer the tin to a wire rack and let the cupcakes cool completely before assembling.
06 - Using an electric mixer, beat the softened butter on medium speed until light and creamy, about 2 minutes. Add the powdered sugar and beat for 1 minute until incorporated. Mix in the marshmallow creme and vanilla extract, beating until the filling is smooth and fluffy. If the consistency is too stiff, add milk one teaspoon at a time until it reaches a pipeable texture.
07 - Once the cupcakes are completely cool, use a small paring knife or cupcake corer to remove a small cone-shaped portion from the center of each cupcake. Reserve the removed tops if desired. Fill a piping bag with the cream filling and pipe it into each cavity. Replace the reserved tops to seal the filling.
08 - In a small saucepan, heat the heavy cream over medium heat until it begins to steam — do not let it boil. Pour the hot cream over the semi-sweet chocolate chips in a heatproof bowl. Let sit undisturbed for 2 minutes, then stir gently until the ganache is glossy and completely smooth.
09 - Spoon the ganache over the top of each filled cupcake, or dip each cupcake top-down into the ganache to coat evenly. Allow the glaze to set at room temperature until firm to the touch.
10 - Stir together the powdered sugar and milk until a smooth, pipeable icing forms, adjusting with additional milk as needed. Transfer to a piping bag fitted with a fine round tip — or use a zip-top bag with a tiny corner snipped off. Pipe the classic curly swirl pattern across the top of each glazed cupcake. Serve at room temperature and enjoy.