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 the wet ingredients into the dry ingredients. Mix gently with a spatula or wooden spoon until just combined. Stop mixing as soon as flour streaks disappear; some small lumps are acceptable and prevent tough muffins.
05 - Fold in 1 3/4 cups chocolate chips, reserving a small handful for topping the muffins.
06 - Divide batter evenly among the 12 muffin cups, filling each cup completely to the top. This creates the signature bakery-style domed tops. Sprinkle reserved chocolate chips over batter.
07 - Bake at 425°F for 5 minutes to create initial rise, then reduce oven temperature to 350°F and continue baking for 13-15 minutes. Muffins are done when 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 before transferring to a wire rack. Cool completely before storing or serving.