Preheat Oven: Preheat to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease lightly.
Mix Dry Ingredients: In a large bowl, whisk together flour, cocoa powder, baking soda, baking powder, and salt.
Mix Wet Ingredients: In another bowl, combine buttermilk, oil, eggs, sugars, and vanilla. Whisk until smooth.
Combine: Pour wet ingredients into dry ingredients and gently fold until just combined. Do not overmix.
Add Chocolate Chips: Fold in chocolate chips, reserving a few for topping.
Fill Muffin Cups: Divide the batter evenly among muffin liners, filling about ¾ full. Sprinkle remaining chips or coarse sugar on top.
Bake: Bake for 18–20 minutes or until a toothpick inserted in the center comes out with a few moist crumbs.
Cool: Let muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.