Preheat the oven to 350°F (180°C) and line a muffin tin with paper liners.
In a medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt.
In a separate bowl, mix the sugar, vegetable oil, egg, and vanilla extract until well combined.
Add the dry ingredients to the wet ingredients, alternating with the buttermilk, and mix until just combined.
Fold in the chocolate chunks.
Divide the batter evenly among the muffin cups, filling each about halfway.
Bake for 15-20 minutes or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the tin for 5 minutes before transferring to a wire rack to cool completely.
In a small saucepan, heat the heavy cream until just simmering.
Pour the hot cream over the chocolate chips and let it sit for 5 minutes.
Stir the mixture until smooth to create the ganache filling.
Once the muffins are completely cool, use a small knife to cut a hole in the center of each muffin and fill it with the ganache.
Enjoy the double chocolate muffins!
