Combine the Puff Mixture: In a large mixing bowl, combine your cold mashed potato, grated Cheddar and Mozzarella, finely sliced spring onions, the single egg yolk, 2 tablespoons of plain flour, smoked paprika, and garlic powder. Season generously with salt and pepper.

Mix Thoroughly: Using a wooden spoon or your hands, mix everything together until it's well combined. Be careful not to overmix, as this can make the potato gummy. The mixture should be firm enough to hold its shape.

Chill the Mixture: Cover the bowl with cling film and place it in the refrigerator to chill for at least one hour. This step is essential as it firms up the mixture, making the puffs much easier to shape and handle. Do not skip this!

Shape the Puffs: Once chilled, take the mixture out of the fridge. Lightly flour your hands and a baking tray. Scoop out portions of the mixture (about a heaped tablespoon each) and roll them into small balls or short, fat cylinders. Place the shaped puffs on the floured tray.

Set Up Your Coating Station: Prepare three shallow dishes. Put the 75g of plain flour in the first, the two beaten eggs in the second, and the panko breadcrumbs in the third.

Coat the Potato Puffs: Working one at a time, gently roll each potato puff first in the flour (tapping off any excess), then dip it into the beaten egg, letting any excess drip off, and finally, roll it in the panko breadcrumbs until it's completely coated. Place the coated puffs back on the tray.

Heat the Oil: Pour the vegetable oil into a large, deep frying pan or skillet until it's about 3-4 cm deep. Heat the oil over a medium-high heat to 180°C (350°F). If you don't have a thermometer, you can test if the oil is ready by dropping in a small breadcrumb; it should sizzle and turn golden in about 30 seconds.

Fry the Puffs: Carefully place the puffs into the hot oil in small batches, ensuring not to overcrowd the pan. Fry for 3-4 minutes, turning them gently, until they are a deep golden brown on all sides and the cheese inside has melted.

Drain and Serve: Using a slotted spoon, remove the cooked mashed potato puffs from the pan and place them on a wire rack to drain any excess oil. This keeps them crispier than using paper towels. Serve them warm.