Preheat your oven to 300°F (148°C) and line a cupcake pan with paper liners.

In a large mixing bowl, combine the flour, sugar, cocoa powder, baking soda, and salt. Whisk them together until evenly mixed and set aside.

In a separate medium bowl, whisk together the egg, milk, vegetable oil, and vanilla extract until well blended.

Pour the wet ingredients into the bowl of dry ingredients and stir until the mixture is fully combined.

Gradually mix in the hot water, stirring until the batter is smooth and very thin in consistency.

Spoon the batter into the cupcake liners, filling each a little over halfway full. Bake for 18 to 23 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.

Remove the cupcakes from the oven and let them cool for 2 minutes in the pan, then transfer them to a wire rack to cool completely.

Once the cupcakes are fully cooled, prepare the filling. Place the butter in the bowl of a mixer and beat it until smooth and creamy.

Add half of the powdered sugar and mix until it’s fully incorporated and smooth. This may take a few minutes.

Add the remaining powdered sugar and continue mixing until smooth.

Add in the marshmallow creme and mix until everything is well combined and fluffy.

To fill the cupcakes, cut a small core from the center of each one using a knife or cupcake corer.

Pipe or spoon the filling into the hollow centers, then replace the top part of the removed cake. You can trim down the piece before replacing it, as you won’t need the entire core with the filling inside.

To prepare the chocolate ganache topping, place the chocolate chips into a medium-sized heatproof bowl.

Heat the heavy cream just until it starts to boil, then pour it over the chocolate chips. Let it sit for 2 to 3 minutes, then whisk until the mixture is smooth and glossy.

Spread the ganache over the top of each cupcake, covering it completely. If the ganache thickens too much as it cools, you can reheat it briefly—5 to 10 seconds in the microwave should make it spreadable again.

To make the decorative vanilla icing, beat the butter in a mixer bowl until smooth.

Add the powdered sugar and mix until it begins to come together—it may be a bit dry and clumpy at first.

Add in the milk and a splash of vanilla extract, then mix until the icing becomes smooth and pipeable.

Transfer the icing to a piping bag fitted with a small round tip (such as Wilton tip #3) and pipe the classic swirl design across the top of each cupcake.

Keep the cupcakes refrigerated until you’re ready to serve. For the best texture and flavor, let them sit at room temperature for a bit before enjoying. Serve cool, but not cold.