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

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

In another bowl, combine the egg, milk, vegetable oil, and vanilla extract.

Mix the wet ingredients into the dry ingredients until just combined.

Gradually stir in the boiling water until the batter is smooth.

Fill each cupcake liner about 2/3 full with batter.

Bake for 18-20 minutes or until a toothpick inserted in the center comes out clean.

Allow cupcakes to cool completely on a wire rack.

For the marshmallow fluff, combine sugar, corn syrup, and water in a small saucepan over medium heat. Stir until the sugar dissolves and the mixture comes to a boil.

In a separate bowl, beat the egg whites until soft peaks form.

Once the sugar mixture reaches 240°F (115°C), slowly pour it into the egg whites while continuing to beat.

Add vanilla extract and beat until the mixture is thick and glossy.

Once the cupcakes are cool, pipe a generous amount of marshmallow fluff on top of each cupcake.

Refrigerate for about 30 minutes to set the fluff.

For the chocolate coating, melt the chocolate chips and vegetable oil together in a microwave-safe bowl in 30-second intervals, stirring in between until smooth.

Dip each cupcake into the melted chocolate, ensuring the marshmallow fluff is completely covered.

Place back on the wire rack to set.