Preheat oven to 400°F (200°C). Grease a muffin tin with nonstick spray or butter.
Mix ingredients: In a large bowl, combine mashed potatoes, egg, cheddar cheese, Parmesan, green onions (if using), garlic powder, salt, and pepper. Stir until fully combined.
Fill muffin tin: Spoon the mixture evenly into the greased muffin cups, filling each about ¾ full.
Bake for 20 - 25 minutes, or until the tops are golden brown and slightly crispy.
Cool and serve: Let the puffs cool in the pan for 5 minutes before removing. Serve warm, with dipping sauce or sour cream if desired.