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 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. Gently fold with a spatula until just combined—batter should remain slightly lumpy. 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 batter.
07 - Bake at 425°F for 5 minutes to create domed tops, then reduce oven temperature to 350°F and continue baking for 13–15 minutes until a toothpick inserted in the center comes out clean or with a few moist crumbs.
08 - Let muffins cool in tin for 5 minutes, then transfer to a wire rack to cool completely.