Preheat oven to 350°F (176°C). Spray a 12-capacity muffin pan with cooking spray. Set aside.
In a large bowl, combine the flour, sugar, baking powder, salt, nutmeg and cinnamon.
In a medium bowl, whisk together the milk, egg, melted butter and vanilla.
Pour the milk mixture into the flour mixture and stir with a rubber spatula until just combined (no large lumps). Batter will be runny.
Spoon batter into muffin pan, filling each cavity 3/4 full. Bake for 20-25 minutes until lightly brown on top, or until a toothpick inserted into the center of a muffin comes out clean.
Allow muffins to rest in the pan for 5 minutes before transferring to a wire rack to cool completely.
Once cooled prepare the cinnamon sugar topping: place the melted butter in one small bowl, and the sugar and cinnamon in a second small bowl. Dip the top and sides of each muffin in the butter first, then dip the muffin in the cinnamon sugar and swirl to coat evenly. Repeat process for all muffins.
Notes
Muffins can be stored in an air-tight container at room temperature for up to 3 days. I like to warm mine in the microwave for 10 seconds each.