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 mixture. 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. Bake for 18 to 20 minutes, or until a toothpick inserted into the center comes out clean.
05 - Transfer the cupcakes to a wire rack and allow them to cool completely before proceeding with assembly.
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 the filling is fluffy and smooth. If the consistency is too thick, add milk one teaspoon at a time until it reaches a pipeable texture.
07 - Using a small knife or cupcake corer, remove a small portion from the center of each cooled cupcake, reserving the removed tops if desired. Transfer the cream filling to a piping bag and fill each cavity. Replace the reserved 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. Stir until the mixture is glossy and completely smooth.
09 - Spoon the ganache over the top of each cupcake or dip the tops 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, adjusting with additional milk as needed. Transfer to a piping bag fitted with a fine tip or a zip-top bag with a small corner snipped off. Pipe the classic squiggly swirl across the top of each cupcake.
11 - Allow the glaze and icing to set before serving. Enjoy at room temperature for the best texture and flavor.