01 - Preheat oven to 425°F. Line a 12-cup muffin tin with paper liners or grease lightly.
02 - Whisk together flour, baking powder, baking soda, and salt in a large bowl until well blended.
03 - Whisk melted butter, oil, granulated sugar, brown sugar, eggs, buttermilk, and vanilla extract in a separate bowl until smooth and fully incorporated.
04 - Pour wet ingredients into dry ingredients. Mix gently until just combined, leaving some lumps. Avoid overmixing to ensure tender muffins.
05 - Fold in chocolate chips, reserving a small handful for topping.
06 - Divide batter evenly among muffin cups, filling each to the top. Sprinkle reserved chocolate chips over each muffin.
07 - Bake at 425°F for 5 minutes. Reduce oven temperature to 350°F and continue baking for 13-15 minutes until a toothpick inserted into the center comes out clean or with a few moist crumbs.
08 - Let muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.