Preheat oven to 400°F (200°C) and lightly grease a mini muffin tin with cooking spray or butter.
In a large mixing bowl, combine mashed potatoes, eggs, cheddar cheese, Parmesan, green onions, garlic powder, salt, and pepper. Mix well until fully combined.
Spoon the mixture evenly into the mini muffin cups, filling them almost to the top.
Bake for 20–25 minutes, or until the tops are golden brown and slightly crispy.
Let cool in the pan for 5 minutes before gently removing with a spoon or butter knife. Serve warm.