Preheat your oven to 400°F (200°C). Grease a baking sheet with cooking spray or oil.
In a large mixing bowl, combine leftover mashed potatoes, shredded cheddar cheese, grated Parmesan cheese, chopped chives (if using), garlic powder, salt, and pepper. Mix until well combined.
Beat the eggs in a separate bowl.
Scoop out spoonfuls of the mashed potato mixture and shape them into small balls or patties.
Dip each ball or patty into the beaten eggs, then roll them in breadcrumbs until coated evenly.
Place the coated potato balls or patties on the prepared baking sheet.
Bake in the preheated oven for 20-25 minutes, or until the puffs are golden brown and crispy.
Serve hot as a tasty appetizer or side dish.