Core the apples and slice them into ½-inch thick rings. Discard the ends.
In a medium bowl, whisk together the flour, 2 tablespoons sugar, and ½ teaspoon cinnamon for the batter.
In a separate bowl, whisk the egg(s), milk, and vanilla extract until combined.
Gradually add the wet ingredients to the dry ingredients, whisking until a smooth, pancake-like batter forms. If it’s too thick, add a little more milk; if too thin, add a bit more flour. Add the second egg if using.
In a shallow dish, combine 1 cup sugar and 2 teaspoons cinnamon for the coating mixture.
Dip each apple ring into the batter, ensuring it’s fully coated. Let any excess batter drip off.
Roll the batter-coated apple ring in the cinnamon-sugar mixture.
Preheat your air fryer to 375°F (190°C).
Arrange the coated apple rings in a single layer in the air fryer basket, working in batches if necessary to avoid overcrowding.
Lightly brush the tops of the apple rings with melted butter.
Air fry for 8-10 minutes, flipping halfway through and brushing the other side with melted butter. The rings should be golden brown and crispy.
Remove from the air fryer and let cool slightly before serving.