Preheat and Prepare: Preheat your oven to 425°F (220°C). Line a large baking sheet with parchment paper and lightly grease with cooking spray.
Mix the Base: In a large mixing bowl, combine cold mashed potatoes, eggs, and flour. Mix until well combined and the mixture holds together when squeezed.
Add Cheese and Seasonings: Fold in 3/4 cup of the cheddar cheese, all the Parmesan cheese, salt, pepper, and chives (if using). Mix until evenly distributed.
Form the Puffs: Using a large spoon or ice cream scoop, portion the mixture into 12 equal mounds on the prepared baking sheet. Shape each mound into a rounded puff shape.
Top with Cheese: Sprinkle the remaining 1/4 cup cheddar cheese and breadcrumbs evenly over each puff, pressing gently to adhere.
Bake: Place in the preheated oven and bake for 18-22 minutes, or until the puffs are golden brown and crispy on the outside.
Cool and Serve: Let cool for 2-3 minutes before serving. Serve hot with your favorite dipping sauces.