Heat oven to 350°. Prepare a 12-cup muffin tin by lining with parchment paper muffin liners.
In a large mixing bowl, combine the flour, sugar, baking powder, cinnamon, and salt. Stir with a wire whisk to break up any clumps.
2 cups all-purpose flour, 1½ cups granulated sugar, 2 teaspoons baking powder, 1 teaspoon ground cinnamon, ½ teaspoon salt
In a smaller bowl combine the milk, egg, melted butter, and vanilla extract. Stir with a whisk until combined.
1¼ cups whole milk, 1 large egg, 2 tablespoons butter, 1 teaspoon vanilla extract
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.
Bake for 18-22 minutes. The muffin tops should be puffy and domed and a toothpick inserted into the center should come out clean.
Remove the muffins from the oven and let them cool in the muffin pan for about 10-15 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.
4 tablespoons butter, 1/3 cup granulated sugar, ¾ teaspoon ground cinnamon
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.