Preheat oven to 350 degrees F. Line a muffin tin with cupcake liners; set aside.
In a medium mixing bowl, stir together the flour, baking powder, and salt; set aside.
In a large mixing bowl, beat together butter and sugar until fluffy, about 1 minute. Add egg and beat to combine.
Add 1/2 of the dry ingredients to the butter mixture and beat together. Once incorporated add in the milk, mix to combine. Add the remaining dry ingredients and mix until just combined.
Use a medium cookie scoop to fill the muffin tin cups 2/3 of the way full. Bake for 15 to 20 minutes, or until a toothpick inserted in the center of a muffin comes out clean.
Remove from oven and allow to cool for 5 minutes, before transferring the muffins to a wire rack to cool completely.
In a small bowl, stir together sugar and cinnamon until evenly combined. In another small bowl, melt butter.
Unwrap the muffins. Dip the top and bottom of a muffin into the butter. Immediately roll the muffin in the cinnamon-sugar mixture. Return to the wire rack. Repeat with remaining muffins. Serve immediately or store in an airtight container.
The nutrition on the recipe is slightly skewed because you will not use all the cinnamon sugar or butter for the rolling process.
I don't dip the side of the muffin because there is usually a little moisture from the muffin itself that helps the cinnamon-sugar stick.