Preheat your oven to 350°F (175°C). Prepare the muffin tin by greasing it or lining it with cupcake liners.

In a large mixing bowl, whisk together the flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt until well combined.

In another bowl, mix the buttermilk, vegetable oil, egg, and vanilla extract. Slowly add this wet mixture to the dry ingredients and mix until just combined.

Carefully stir in the boiling water to the batter. The batter will be thin, but this is normal.

Pour the batter into the prepared muffin tin, filling each cup about two-thirds full.

Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Allow the cupcakes to cool in the tin for about 5 minutes, then transfer them to a cooling rack to cool completely.

While the cupcakes are cooling, prepare the filling by beating together the marshmallow fluff, powdered sugar, softened butter, and vanilla extract until smooth.

Once the cupcakes are cool, use a small knife to cut out the center from each cupcake. Fill the center of each cupcake with the marshmallow filling using a piping bag.

For the ganache, heat the heavy cream in a saucepan over medium heat until just simmering. Remove from heat and pour over the chocolate chips in a separate bowl. Let sit for a minute, then stir until smooth.

Dip the tops of each filled cupcake into the ganache or pour it over the cupcakes for an even coating.

Optional: Use the remaining marshmallow filling and pipe a swirl on each ganache-covered cupcake.