Heat oven to 350°. Prepare a 12-cup muffin tin by spraying it really well with cooking spray. You can also use a 24-cup mini muffin tin as well.
In a large mixing bowl, combine the flour, sugar, baking powder, cinnamon, and salt. Stir with a wire whisk to break up any clumps.
In a smaller bowl combine the milk, egg, melted butter, and vanilla extract. Stir with a whisk until combined.
Pour the wet ingredients into the dry ingredients and stir together with a spatula or wooden spoon until combined. You don't want to overmix but you also want to make sure that you mix it enough so no flour pockets or streaks remain in the batter. Fill each muffin cup 3/4 full with the batter.
Cook regular sized muffins for 18-22 minutes. Cook mini-sized muffins for 12-14 minutes. Let the muffins cool in the pan for about 10 minutes or until you're able to handle them without burning yourself for dipping.
Cinnamon Sugar Topping : Place melted butter in a small bowl. Mix together the sugar and cinnamon in another small bowl. Dip the tops of the muffins into the butter and then immediately into the cinnamon sugar topping.
Place dipped muffins on a cooling rack and repeat with the remaining muffins. Let cool completely or eat them warm.
Store leftovers, at room temperature, in a covered container or a gallon-sized Ziploc bag.