Combine the Potato Mixture: In a large mixing bowl, combine your cold mashed potatoes, the grated Cheddar and Parmesan, the two egg yolks, chopped chives, and 50g of plain flour. Season generously with salt, black pepper, and a small pinch of nutmeg.

Mix and Chill: Use a wooden spoon or your hands to mix everything together until just combined. Be careful not to overwork the mixture, as this can make the potatoes gummy. Cover the bowl with cling film and place it in the refrigerator to chill for at least one hour. This step is vital for making the mixture firm and easy to handle.

Shape the Puffs: Once chilled, lightly flour your hands and a clean work surface. Take a tablespoon of the potato mixture and roll it into a neat ball, roughly the size of a golf ball. Place the shaped puff onto a tray lined with baking parchment and repeat with the remaining mixture.

Set Up Your Breading Station: Prepare three shallow dishes. Place some plain flour in the first, the two remaining eggs (lightly beaten) in the second, and the panko breadcrumbs in the third.

Coat Each Puff: Working one at a time, gently roll each potato ball first in the flour (tapping off any excess), then dip it into the beaten egg, and finally, roll it in the panko breadcrumbs until it's completely and evenly coated. Place the coated puffs back onto the lined tray.

Heat the Oil: Pour the vegetable oil into a large, deep frying pan or a Dutch oven until it's about 5cm deep. Heat the oil over a medium-high heat until it reaches 170-180°C (340-350°F). If you don't have a thermometer, you can test it by dropping a small piece of bread into the oil; it should turn golden brown in about 30-40 seconds.

Fry in Batches: Carefully lower 4-5 puffs into the hot oil using a slotted spoon. Do not overcrowd the pan. Fry for 3-4 minutes, turning them gently, until they are deep golden brown and crisp all over.

Drain and Serve: Remove the cooked puffs from the oil with the slotted spoon and let them drain on a wire rack. This helps them stay crispy. Sprinkle with a little extra salt while they are still hot. Repeat with the remaining puffs and serve immediately.