Preheat oven to 425˚F (220˚C). Line 12 muffin tins with liners or lightly grease.
In a large bowl, whisk together the flour, sugar, baking powder, baking soda, salt, and nutmeg.
Make a well in the center and add the buttermilk, vegetable oil, eggs, and vanilla.
Whisk together just until combined without any lumps.
Gently fold in the mini chocolate chips.
Transfer the batter into each prepared muffin tin until 3/4 full. If desired, top each muffin with a sprinkling of vanilla sugar or coarse sugar.
Bake in preheated oven for 5 minutes, then reduce heat to 350˚F (180˚C) and bake for another 10-15 minutes, until a toothpick inserted in the center comes out clean (melted chocolate is fine). It will be closer to 12 minutes for regular-sized muffins and 15 minutes for large.
Cool in the pan for 5 minutes before removing to a wire rack.
These muffins are best served the day they are made, especially within a couple of hours of baking.