Chocolate Chip Muffins

0 Heat butter in the microwave or on the stovetop until just melted. Set aside to cool slightly (about 10 minutes).
1 Preheat oven to 350°F / 177°C.
2 Spray a muffin pan with nonstick cooking spray (or line with paper liners).
3 In a large mixing bowl, whisk together flour, sugar, baking powder, salt, and cinnamon. Set aside. 
4 In a second, smaller mixing bowl, whisk together sour cream, egg, vanilla, and melted butter. Slowly pour in milk while whisking.
5 Pour wet ingredients over dry ingredients and stir just until no dry spots remain. (Do not overmix.) 
6 Gently fold in chocolate chips.
7 Scoop batter into prepared muffin pan. Top each muffin with a few extra chocolate chips and turbinado sugar (if using). 
8 Bake until muffins are golden on top and a tester inserted in the middle comes out clean, 21 to 24 minutes.
9 Allow muffins to cool slightly in the pan and then transfer to a wire rack to finish cooling.