Preheat the oven to 400°F (200°C) and lightly grease a baking sheet with cooking spray or oil.
In a large bowl, combine the cooled mashed potatoes, cheddar cheese, Parmesan cheese, green onions, egg, garlic powder, onion powder, black pepper, and salt. Mix until well combined.
Using your hands, form the potato mixture into small balls, about 1 inch in diameter.
Roll each ball in breadcrumbs until fully coated and place them on the prepared baking sheet.
Bake in the preheated oven for 20-25 minutes, or until the bites are golden brown and crispy.
Remove from the oven and let cool for a few minutes before serving.