Preheat your 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, flour, egg, garlic powder, onion powder, black pepper, salt, and green onions (if using). Mix until well combined.
Using your hands or a spoon, scoop out about 2 tablespoons of the potato mixture and form it into a small ball or puff shape. Repeat with the remaining mixture.
Roll each puff in breadcrumbs to coat evenly and place them on the prepared baking sheet.
Bake in the preheated oven for 20-25 minutes, or until the puffs are golden brown and crispy on the outside.
Remove from the oven and let cool for a few minutes before serving.