Preheat the oven to 350°F. Line a standard 12-cup muffin pan with paper liners.
In a large mixing bowl, combine the flour, granulated sugar, baking powder, salt, cinnamon, and nutmeg. Whisk together until the dry ingredients are evenly combined.
In a medium bowl, whisk together the milk, egg, melted butter, and vanilla until smooth.
Pour the milk mixture into the dry ingredients. Stir gently with a wooden spoon or spatula just until combined and no large lumps remain. The batter will be slightly runny.
Using a spoon or cookie scoop, fill each muffin cup almost to the top with batter.
For standard-sized muffins, bake for 20-24 minutes, or until a toothpick inserted into the center comes out clean.
Remove the muffins from the oven and let them rest in the pan for 5 minutes. Then transfer the muffins to a wire rack to cool slightly; they should still be warm for the topping step.
While the muffins are still warm, prepare the topping. In a small bowl, stir together the sugar and cinnamon.
Dip the top of each muffin into the melted unsalted butter, coating it fully.
Immediately roll or dip the buttered muffin top into the cinnamon-sugar mixture, pressing lightly so it sticks.
Place the finished muffins back on the wire rack. Repeat with the remaining muffins. Serve warm or allow to cool completely.