In a bowl, combine the cold risotto, parmesan cheese, and 1 beaten egg. Mix well until everything holds together. If using plain arborio rice, add an extra tablespoon of parmesan to help it bind.
Take about 2 tablespoons of the rice mixture, flatten it in your palm, and place a mozzarella cube in the center. Close the rice around the cheese, shaping it into a firm ball. Repeat to make about 8 arancini.
Place them on a plate and refrigerate for 15 minutes so they firm up.
Set up a breading station. Place the flour in one bowl, the beaten eggs in a second bowl, and the panko breadcrumbs mixed with garlic powder and salt in a third bowl. Roll each arancini in flour, dip in egg, then coat thoroughly in the panko mixture, pressing gently.
Preheat the air fryer to 375°F (190°C) for 3 minutes. Spray the basket with cooking spray.
Place the arancini in the basket in a single layer with space between each one. Spray all sides generously with cooking spray.
Air fry at 375°F for 12 to 14 minutes, turning them gently every 4 minutes, until deeply golden and crispy all over.
Let them rest for 2 minutes, then serve with warm marinara sauce. The mozzarella inside will be perfectly melted and stretchy.