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