Preheat oven to 425 degrees F. Spray a muffin tin with cooking spray, both within the cups and the surface in between. Line muffin cups with liners. Set aside.
In a large bowl, whisk together flour, baking powder, baking soda, sugar, cinnamon, nutmeg, and salt. Once combined, mix in chocolate chips. Set aside.
In a small bowl, whisk together melted & cooled butter, vanilla extract, eggs, and buttermilk until egg yolks are broken and blended. Mixture may be lumpy - this is okay.
Pour wet ingredients into dry ingredients. Use a spatula to stir, scraping along the bottom and pressing down on top. Stop mixing as soon as the dry ingredients have all been moistened. Batter should be very thick (like cookie dough) and a little lumpy.
Fill each cup in the muffin tin to be completely full, roughly 1/4 to 1/3 cup of muffin mix. If desired, press extra chocolate chips on top of the batter and sprinkle with coarse sugar.
Bake at 425 F for 5 minutes. Reduce heat to 375 degrees F (I like to open and close the oven door a few times) and bake for another 11-13 minutes or until tops of muffins are puffed and golden brown.
Let muffins cool for a few minutes before removing them from the muffin tin. Serve immediately.