01 - Preheat oven to 425°F. Line a 12-cup muffin tin with paper liners or lightly grease with cooking spray.
02 - In a large mixing bowl, whisk together flour, baking powder, baking soda, and salt until well blended.
03 - In a separate bowl, whisk together melted butter, vegetable 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 or wooden spoon until just combined. Do not overmix—small lumps are acceptable and prevent tough muffins.
05 - Fold in chocolate chips, reserving a small handful for topping. Distribute chips evenly throughout batter.
06 - Divide batter evenly among prepared muffin cups, filling each to the top. For bakery-style domed tops, mound batter slightly in center. Sprinkle reserved chocolate chips on top.
07 - Bake at 425°F for 5 minutes to create rise, then reduce oven temperature to 350°F and continue baking for 13-15 minutes. Muffins are done when toothpick inserted in center comes out clean or with few moist crumbs.
08 - Let muffins cool in tin for 5 minutes, then carefully transfer to wire rack. Cool completely before storing or serving.