Preheat your oven to 400°F. Grease a baking sheet with cooking spray or oil.
In a large bowl, combine the cooled mashed potatoes, cheddar cheese, Parmesan cheese, sour cream, green onions, egg, garlic powder, onion powder, salt, and black pepper. Mix until well combined.
Using your hands, scoop out about 2 tablespoons of the potato mixture and form it into a small ball or puff shape. Repeat with the remaining mixture and place the puffs on the prepared baking sheet.
Roll each puff in breadcrumbs until fully coated, then return them to the baking sheet.
Bake in the preheated oven for 20-25 minutes, or until the puffs are golden brown and crispy.
Serve warm as a delicious appetizer or side dish.