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, salt, and black pepper. Mix until well combined.
If using, fold in the chopped green onions for added flavor.
Using a spoon or cookie scoop, portion out the mixture and shape it into small balls or mounds, placing 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 slightly before serving.