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

In a mixing bowl, combine flour, sugar, cocoa powder, baking powder, baking soda, and salt.

In another bowl, whisk together the egg, milk, oil, and vanilla extract.

Pour the wet ingredients into the dry ingredients and mix until just combined.

Divide the batter into two bowls. Add additional cocoa powder to one bowl to create a darker batter.

Spoon alternating dollops of the two batters into each cupcake liner, then use a toothpick to swirl them together.

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

Remove from the oven and let cool in the pan for 5 minutes before transferring to a wire rack to cool completely.

In a mixing bowl, beat the chilled heavy cream until soft peaks form.

Gradually add powdered sugar and cocoa powder, continuing to beat until stiff peaks form.

Mix in vanilla extract until combined.

Once the cupcakes are completely cool, use a piping bag to fill each cupcake with the creamy chocolate filling.

Serve and enjoy your swirled chocolate cupcakes!