Preheat the oven to 350°F (175°C). Line a standard 12-cup muffin tin with paper cupcake liners and set aside.

In a large mixing bowl, whisk together the flour, granulated sugar, cocoa powder, baking soda, and salt. In a separate bowl, combine the egg, milk, vegetable oil, and vanilla extract. Gradually incorporate the wet ingredients into the dry ingredients, stirring until just combined. Add the hot coffee or water and mix until the batter is smooth and evenly blended.

Distribute the batter evenly among the cupcake liners, filling each approximately two-thirds full. Bake for 18 to 20 minutes or until a toothpick inserted into the center emerges clean. Allow the cupcakes to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.

In a medium bowl, beat the softened butter until creamy. Gradually add the powdered sugar and continue to beat until light and fluffy. Fold in the marshmallow crème and vanilla extract until fully incorporated. Transfer the filling to a piping bag fitted with a small round tip.

Place the chocolate chips in a heatproof bowl. In a small saucepan, bring the heavy cream to a gentle simmer over medium heat. Pour the hot cream over the chocolate and allow it to sit for 2 minutes. Stir until the mixture becomes smooth and glossy. Set aside to cool slightly.

Use a small knife to cut a cone-shaped piece from the center of each cooled cupcake. Discard or reserve the cut-out portions. Fill each cavity with the marshmallow filling until level with the surface of the cupcake.

Gently dip the tops of the filled cupcakes into the prepared ganache, ensuring an even coating. Place the cupcakes back on the wire rack and allow the ganache to set at room temperature.

In a small bowl, whisk together the powdered sugar, milk, and vanilla extract until smooth and thick. Transfer the icing to a piping bag fitted with a fine tip. Pipe the signature swirl or decorative design atop each cupcake once the ganache has fully set.