Preheat oven to 350 F. In a large bowl, whisk together flour, sugar, salt, baking powder, and cinnamon. Add in egg, milk, and melted butter. Whisk until smooth. Evenly divide batter between 12 greased muffin cups.
Bake for 25-30 minutes. Place melted butter n a small bowl, whisk together sugar and cinnamon in another bowl. Dip each muffin top into the melted butter, then into the cinnamon and sugar.