Preheat your oven to 350°F (175°C). Line a muffin tin with cupcake liners.

In a large bowl, combine the flour, sugar, cocoa powder, baking powder, baking soda, and salt. Whisk until well blended.

Add the egg, milk, vegetable oil, and vanilla extract to the dry ingredients. Mix until just combined. Gradually stir in the boiling water until the batter is smooth.

Pour the batter into the prepared muffin tin, filling each liner about 2/3 full. Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Let the cupcakes cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.

While the cupcakes cool, prepare the filling. In a medium bowl, beat the softened butter until creamy. Gradually add the powdered sugar, marshmallow fluff, and vanilla extract, mixing until smooth and fluffy.

Once the cupcakes are cool, use a small knife or cupcake corer to remove a small portion from the center of each cupcake. Fill each hole with the marshmallow filling.

For the ganache, heat the heavy cream in a small saucepan over medium heat until it just begins to simmer. Remove from heat and add the chocolate chips and butter. Stir until the chocolate is melted and the mixture is smooth.

Dip the top of each filled cupcake into the ganache, allowing any excess to drip off. Let the ganache set for about 30 minutes before serving.