Prep. Preheat oven to 375˚F. Line a muffin pan with paper liners.
Grate the apples. Grate the apples using the large holes of a box grater. Transfer grated apples to a strainer; cover with plastic wrap and let stand to drain.
Mix the dry ingredients. In a mixing bowl combine flour, sugar, baking powder, cinnamon, and salt; whisk until incorporated. Set aside.
Mix the wet ingredients. Pour melted butter into a mixing bowl. Let cool slightly. Whisk in the egg and then the milk.
Combine. Whisk 1/3 of the flour mixture into the milk mixture. Whisk in another 1/3 of the flour mixture into the liquid mixture; whisk until the liquid is absorbed. Whisk in the remaining of the flour mixture. Gently fold in the grated apples.
Fill the pan. Evenly distribute the batter into the prepared muffin pan. Set aside.
Top with cinnamon sugar. In a small bowl combine sugar and cinnamon; mix to incorporate. Sprinkle the tops of the muffin batter with cinnamon sugar.
Bake. Bake muffins at 375ºF for 20 minutes, or until risen, golden on top, and a toothpick inserted in the centers comes out dry. Remove from oven.
Cool. Cool muffins in the pan set on a rack. Lift them out when they have cooled completely.
Notes
TO FREEZE: Wrap each completely cooled muffin individually in plastic wrap and freeze. Bring to room temperature before serving.