Prep oven and pan. Position a rack in the center of the oven and preheat to 425°F (218°C). Grease a muffin tin with either butter or non-stick cooking spray or line with muffin liners.
Combine dry ingredients. In a large mixing bowl, whisk together the flour, cornstarch, sugar, baking powder, and salt together. 2 ¼ cups (270 g) all-purpose flour 2 teaspoons cornstarch 1 cup (200 g) granulated sugar 1 tablespoon baking powder ¼ teaspoon salt
Combine the wet ingredients. Whisk the melted butter, oil, buttermilk, vanilla and eggs together in a medium bowl. ⅓ cup (5 1/3 Tablespoons, 75 g) unsalted butter ⅓ cup (80 ml) vegetable oil ½ cup + 1 tablespoon (135 ml) buttermilk 2 large eggs 1 ½ teaspoons vanilla extract
Combine wet and dry ingredients. Add the flour mixture to the oil mixture and stir together with a spatula or spoon until just combined. Do not over mix, the batter may be slightly lumpy. Fold in the chocolate chips. 1 cup (170 g) mini chocolate chips
Scoop and bake. Using a 2 1/2-inch diameter ice-cream scoop, portion the batter, rounded side up, into the prepared muffins cups. Fill the muffin cups all the way to top (even slightly mounded over). Bake for 5 minutes. Reduce the oven temperature to 350°F (180°C) and continue baking for 15 minutes more until the tops of the muffins are golden brown and toothpick inserted into the center of a muffin comes out clean or with a few moist crumbs. Cool in the pan for 10 minutes. Remove the muffins from the pan and cool completely.