Prepare the Potato Mixture: In a large mixing bowl, combine the cold mashed potatoes, cheddar cheese, Parmesan cheese, flour, eggs, chives (if using), garlic powder, salt, and pepper. Mix well until all ingredients are thoroughly incorporated. The mixture should be thick and slightly sticky.

Chill the Mixture: Cover the bowl with plastic wrap and refrigerate for at least 30 minutes, or up to 2 hours. This chilling time is crucial as it allows the mixture to firm up, making it easier to handle and preventing the puffs from spreading too much during cooking.

Shape the Puffs: Remove the potato mixture from the refrigerator. Using an ice cream scoop or a spoon, scoop out portions of the mixture and gently roll them into 1-inch balls. Place the balls on a baking sheet lined with parchment paper (if baking) or a plate (if frying).

Choose Your Cooking Method: Frying: Heat about 2 inches of vegetable oil in a deep fryer or large pot to 350°F (175°C). Carefully drop the potato puffs into the hot oil, a few at a time, ensuring not to overcrowd the fryer. Fry for 2-3 minutes, or until golden brown and crispy, flipping occasionally for even cooking. Remove the puffs with a slotted spoon and place them on a paper towel-lined plate to drain excess oil.

Baking: Preheat your oven to 400°F (200°C). Place the shaped potato puffs on a baking sheet lined with parchment paper. Lightly spray the puffs with cooking spray. Bake for 20-25 minutes, or until golden brown and slightly crispy, flipping halfway through for even browning.

Serve: Serve the mashed potato cheese puffs immediately while they are still hot and crispy. They can be enjoyed plain or with your favorite dipping sauce, such as ranch dressing, ketchup, or a spicy aioli.