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

In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, and salt. Set aside.

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

Add the wet ingredients to the dry ingredients and mix until just combined. The batter will be thick.

Gradually stir in the hot water until the batter is smooth and pourable. It will be thin.

Fill cupcake liners about two-thirds full. Bake for 18-23 minutes, or until a toothpick inserted into the center comes out with moist crumbs.

Let cupcakes cool in the pan for a few minutes before transferring to a wire rack to cool completely.

While cupcakes cool, make the filling. Beat the room temperature butter in a mixer bowl until smooth.

Add half of the powdered sugar and mix until smooth, scraping down the bowl as needed. Add the remaining powdered sugar and mix until well combined.

Add the marshmallow creme and stir until fully incorporated and smooth.

To fill the cupcakes, use a cupcake corer or a small knife to remove the center of each cupcake. Be careful not to go all the way to the bottom.

Spoon or pipe the filling into the hollowed-out centers of the cupcakes. Place the removed cake pieces back on top of the filling.

To make the ganache, place semi-sweet chocolate chips in a heatproof bowl. Heat heavy whipping cream in a saucepan just until it begins to simmer. Pour the hot cream over the chocolate chips. Let stand for 3-5 minutes, then whisk until smooth and glossy.

Spread the ganache over the tops of the filled cupcakes, allowing it to drip down the sides slightly.

For the vanilla icing, beat the room temperature butter until smooth. Add powdered sugar and mix until crumbly. Add milk and vanilla extract and beat until smooth and creamy.

Transfer the icing to a piping bag fitted with a small round tip. Pipe a decorative swirl on top of each ganache-covered cupcake.

Refrigerate the cupcakes until ready to serve for the best texture.