01 - Preheat oven to 425°F. Line a 12-cup muffin tin with paper liners or grease lightly.
02 - In a large bowl, whisk together flour, baking powder, baking soda, and salt until well blended.
03 - In a separate bowl, whisk together melted butter, oil, granulated sugar, brown sugar, eggs, buttermilk, and vanilla extract until smooth and fully incorporated.
04 - Pour wet ingredients into dry ingredients. Mix gently with a spatula until just combined, being careful not to overmix. Small lumps are acceptable.
05 - Gently fold in chocolate chips, reserving a small handful for topping the muffins.
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 to create tall muffin tops.
08 - Reduce oven temperature to 350°F and continue baking for 13-15 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
09 - Let muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.