Preheat your oven to 400 degrees F (200 degrees C) and grease a mini muffin pan with cooking spray or butter.
In a large bowl, mix together the chilled mashed potatoes, egg, shredded cheddar, chives, flour, garlic powder, salt, and pepper until well combined.
Roll the mixture into 1-inch balls, then roll each ball in grated Parmesan cheese to coat.
Place the coated potato balls into the cavities of the prepared muffin pan, gently pressing down to form a puff shape.
Bake for 15-20 minutes, or until the puffs are golden brown and crisp on the outside.
Let them cool for a few minutes before removing them from the pan. Serve warm and enjoy watching them disappear!
