Preheat the oven to 425° Fahrenheit and line a muffin tin with paper liners or lightly grease it with nonstick spray.
Whisk together the flour, baking powder, baking soda, salt, and sugar in a large bowl.
In a separate bowl, whisk the melted butter, vegetable oil, eggs, buttermilk, and vanilla extract until well combined.
Pour the wet ingredients into the dry ingredients and gently fold with a spatula until just combined. Do not over mix. Fold in the mini chocolate chips, reserving a handful to sprinkle on top..
Use an ice cream scoop or spoon to fill each muffin cup all the way to the top. This is key for big, fluffy muffins.
Sprinkle the tops with extra mini chocolate chips and a generous pinch of coarse sugar for a bakery-style look.
Bake at 425° Fahrenheit for 5 minutes, then reduce the temperature to 350° Fahrenheit and bake for another 15–17 minutes until golden brown and a toothpick inserted in the center comes out clean.
Allow the muffins cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.