Prepare apple cubes:
Remove apple peel and cut into small cubes. Combine with cinnamon sugar in a bowl and reserve.Make batter:
Whisk together egg, sugar, and butter until light and airy. Slowly incorporate flour, baking powder, and salt, stirring thoroughly.Fold in apples:
Gently mix in the apple chunks. Portion the batter evenly into 5 muffin cups.Air fry muffins:
Set the muffin cups in the air fryer basket and cook at 160°C (320°F (160°C)) for 15-17 minutes. Insert a toothpick to check for doneness.