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