Preheat the oven to 350°F (175°C) and grease a 12-cup muffin pan.
Whisk flour, baking powder, salt, cinnamon, and sugar.
Mix the egg, milk, melted butter, and vanilla separately.
Fold the wet mixture into the dry ingredients.
Divide the batter evenly among the muffin cups.
Bake for 16–19 minutes until lightly golden.
Mix the coating sugar and cinnamon in a shallow bowl.
Brush warm muffins with melted butter and roll them in cinnamon sugar.