Preheat oven to 350°F.
In a large mixing bowl, combine flour, sugar, baking powder, salt and cinnamon.
In a separate bowl, whisk together milk, egg, melted butter, and vanilla.
Pour the milk mixture into the flour mixture and stir just until no big lumps remain. The batter will be a bit runny.
Fill prepared muffin pan about ⅔ full of batter per spot. For mini muffins bake 12-14 minutes, for full-size muffins, bake 20-24 minutes.
Remove from oven and let rest in pan for 5 minutes before placing the muffins on a rack to cool.
Once cooled, combine sugar and cinnamon topping. Dip tops of muffins in the melted butter then roll the top in the cinnamon sugar to coat. Place back on the rack and repeat with remaining muffins.