1. Preheat oven to 210°C/400°F. Line a muffin pan with cupcake liners.
2. To make buttermilk mix ¼ cup of warm milk with vinegar and set aside for 2-3 minutes.
3. Whisk the flour, sugar, cocoa powder, baking powder, salt, and mixed chocolate chunks together in a large bowl. Set aside.
4. Whisk the eggs, buttermilk, oil, milk, and vanilla extract together until combined. Pour wet ingredients into dry ingredients and fold them together with a rubber spatula or wooden spoon until completely combined.
5. Cover and let it chill in the fridge for at least 1 hour and at max overnight.
6. Spoon the batter into liners, filling them all the way to the top.
7. Bake for 5 minutes at 210°C/400°F.Then, keeping the muffins in the oven, reduce the oven temperature to 180°C /350°F. Bake for an additional 15-16 minutes or until a toothpick inserted in the center comes out with a few moist crumbs.
8. Cool muffins for 10 minutes in the pan, then transfer to a wire rack until ready to eat.
