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 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 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 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 powdered sugar and beat for 1 minute. Mix in marshmallow creme and vanilla extract, beating until fluffy. If the filling is too thick, 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. Fill each cavity with cream filling using a piping bag. Replace the removed tops if desired.
08 - Heat the heavy cream in a small saucepan until steaming but not boiling. Pour 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 is set.
10 - Stir powdered sugar and milk together 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 decorative swirls across the top of each cupcake.
11 - Serve at room temperature. Pair with a glass of cold milk or a robust coffee for the best experience.