Combine the Ingredients: In a large mixing bowl, combine the cold mashed potato, grated cheddar, grated mozzarella, plain flour, beaten egg, chopped chives, garlic powder, and mustard powder. Add a pinch of nutmeg and season generously with salt and pepper.

Mix Thoroughly: Use a wooden spoon or your hands to 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.

Shape the Puffs: Lightly dust your hands with a little plain flour. Take about a tablespoon of the potato mixture and roll it between your palms to form a small, smooth ball, roughly the size of a golf ball. Place the formed ball on a plate or baking tray lined with parchment paper. Repeat with the remaining mixture.

Chill (Optional but Recommended): For the best results, place the tray of potato puffs in the refrigerator to chill for at least 15-20 minutes. This helps them firm up even more and reduces the chance of them falling apart during cooking. What works best for me is this chilling step; I never skip it.

Heat the Oil: Pour the vegetable oil into a deep, heavy-bottomed frying pan or saucepan until it's about 5cm deep. Heat the oil over a medium-high heat to 180°C. If you don't have a thermometer, you can test if the oil is ready by dropping a small cube of bread into it – it should turn golden brown in about 30 seconds.

Fry the Puffs: Carefully place the puffs into the hot oil in batches, being sure not to overcrowd the pan. Fry for 3-4 minutes, turning them gently with a slotted spoon, until they are deep golden brown and crisp all over.

Drain and Serve: Use a slotted spoon to remove the cooked puffs from the oil and transfer them to a plate lined with kitchen paper to drain off any excess oil. Serve immediately while they are hot and the cheese is gooey.