Combine the Ingredients: In a large bowl, place your cold mashed potato. Add the grated cheddar, egg yolks, plain flour, chopped chives, mustard powder, and garlic powder. Season generously with salt and black pepper.

Mix Thoroughly: Using a wooden spoon or a spatula, mix everything together until it's well combined. Be careful not to overmix, as this can make the potato gummy. The mixture should be thick and slightly sticky.

Chill the Mixture: Cover the bowl with cling film and place it in the refrigerator for at least 30 minutes, or up to 2 hours. I find this step is absolutely essential. Chilling the mixture makes it much less sticky and far easier to handle and shape.

Preheat and Prepare: When you're ready to bake, preheat your oven to 200°C (180°C fan). Line a large baking tray with baking parchment.

Shape the Puffs: Lightly flour your hands. Take about a tablespoon of the chilled mixture and roll it between your palms to form a small ball, about the size of a golf ball. Place it on the prepared baking tray. Repeat with the remaining mixture, spacing the puffs about 2-3 cm apart.

Bake to Golden Perfection: Brush the tops of the puffs with the olive oil or melted butter. This will help them get extra crispy and golden. Bake in the preheated oven for 20-25 minutes, or until they are puffed up, golden brown, and crisp on the outside.

Serve Immediately: For the best texture, serve the cheesy mashed potato puffs while they are still warm. The cheese will be gloriously melty in the centre.