Begin by preheating your oven to 400 degrees F. Spray a 12-cup muffin pan with baking spray or line the cups with baking papers, then spray with baking spray.
In a large bowl, whisk together the flour, sugar, baking powder, baking soda, salt, chocolate chips, and cinnamon. Set aside.
In a medium bowl, whisk together the milk, eggs, sour cream, vegetable oil, and vanilla extract.
Pour the wet ingredients into the dry ingredients. Fold the two together gently. Mix until just barely/almost combined. This is the key to light and tender muffins. Do not overmix.
Spoon the batter into each muffin paper until about 3/4ths of the way full. Sprinkle with coarse sugar.
Bake for 16-20 minutes or until a toothpick inserted into the middle of the muffin comes out clean.
Remove the muffins from the pan when they are cool enough to handle. Allow them to finish cooling on a wire rack.