Set your air fryer to 175°C (350°F).
In a large bowl, whisk together the flour, sugar, baking powder, cinnamon, nutmeg, and salt.
In another bowl, whisk the milk, egg, vanilla extract, and melted butter until well combined.
Gradually add the wet ingredients to the dry ingredients, mixing until just combined. Fold in the diced apple pieces
Lightly spray a donut pan with cooking spray. Spoon the batter into the pan, filling each mold about 3/4 full
Air Fry: Place the donut pan in the air fryer and cook for 10-12 minutes, or until the donuts are golden brown and a toothpick inserted comes out clean
Topping: While the donuts are still warm, brush them with melted butter and dip them into the cinnamon-sugar mixture until well coated.