In a large bowl, combine the leftover mashed potatoes, cheddar cheese, Parmesan cheese, flour, egg, garlic powder, onion powder, salt, and black pepper. Mix until well combined.
Using your hands, form the mixture into small balls, about 1 to 1.5 inches in diameter. Place the balls on a baking sheet lined with parchment paper.
Once all the balls are formed, roll each ball in breadcrumbs until fully coated.
Preheat your air fryer to 375°F (190°C) for about 5 minutes.
Spray the air fryer basket with cooking spray. Place the potato balls in the basket in a single layer, making sure they are not touching.
Air fry the potato balls for 10-12 minutes, or until they are golden brown and crispy, shaking the basket halfway through cooking for even browning.
Remove the potato balls from the air fryer and let them cool for a few minutes before serving.