Preheat Oven:
Preheat oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners or lightly grease.
Mix Dry Ingredients:
In a large bowl, whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt.
Mix Wet Ingredients:
In a separate bowl, whisk together buttermilk, oil, eggs, and vanilla until combined.
Combine Ingredients:
Pour wet ingredients into dry ingredients and gently fold together until just combined. Do not overmix.
Add Chocolate Mix-ins:
Fold in semi-sweet chocolate chips and milk chocolate chunks.
Fill Muffin Tin:
Divide batter evenly among 12 muffin cups. Sprinkle mini chocolate chips on top if desired.
Bake:
Bake for 18–22 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Cool:
Allow muffins to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.