Preheat the oven to 425°F (220°C). Spray a 6-cup muffin pan with nonstick spray or line it with cupcake liners, then set it aside.
In a large bowl, whisk together the butter, sugar, egg, vanilla, sour cream, and milk until well combined.
Add the flour, baking powder, baking soda, salt, and mini chocolate chips and fold together gently until just combined.
Spoon the batter into the prepared muffin pan, filling each cup to the top; you’ll use 4½ tablespoons of batter per cup. Sprinkle the tops with turbinado sugar, if using.
Bake the muffins for 5 minutes. Keeping the muffins in the oven, reduce the oven temperature to 350°F (180°C), and bake them for 20 to 22 minutes, or until a toothpick inserted into the center comes out clean. Allow the muffins to cool for 10 minutes in the pan before removing them to a wire rack to cool completely.