1. Preheat oven to 425°F (220°C). Grease a muffin pan or line with muffin liners.
2. In a large bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg if using.
3. In a separate bowl, whisk melted butter, vegetable oil, sugar, and eggs until combined. Add sour cream, milk, and vanilla extract, mixing gently until smooth.
4. Pour the wet ingredients into the dry ingredients and fold carefully with about 20 strokes—avoid overmixing to keep batter thick and muffins tender.
5. Fold in chocolate chips evenly.
6. Fill muffin cups all the way to the top to encourage tall, domed tops. You may fill every cup or every other cup depending on your muffin pan size.
7. If desired, sprinkle the top with coarse or sanding sugar for texture and sparkle.
8. Bake at 425°F for 5 to 8 minutes, then reduce heat to 350°F (175°C) and bake for an additional 15 to 26 minutes until the muffins are golden and a toothpick inserted in the center comes out clean.
9. Cool muffins in the pan for 10 minutes before transferring to a wire rack to cool further.