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

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

Add the egg, milk, vegetable oil, and vanilla extract to the dry ingredients and mix until combined.

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

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

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

Allow the cupcakes to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.

In a small saucepan, heat the heavy cream over medium heat until it begins to simmer.

Remove from heat and add the chopped chocolate, stirring until melted and smooth.

Stir in the powdered sugar until fully combined.

Let the filling cool to room temperature, then refrigerate until thickened, about 30 minutes.

Once the cupcakes are completely cool, use a knife or cupcake corer to remove a small portion from the center of each cupcake.

Fill a piping bag with the chocolate filling and pipe it into the center of each cupcake.

Top with additional cupcake batter if desired and swirl gently with a toothpick for a marbled effect.

Serve and enjoy your swirled chocolate cupcakes!