Step 1: In a large bowl, combine the chilled mashed potatoes, beaten egg, shredded cheddar cheese, all-purpose flour, chopped chives (or green onions), garlic powder, salt, and pepper. Mix thoroughly until all ingredients are well combined.
Step 2: Using a small scoop or your hands, form the potato mixture into 1.5-inch balls or small patties. Ensure they are compact so they don't fall apart during frying.
Step 3: Heat about 2-3 inches of vegetable oil in a deep pot or Dutch oven over medium-high heat to 350-375°F (175-190°C). If you don't have a thermometer, a small drop of batter should sizzle immediately and float.
Step 4: Carefully place a few potato puffs into the hot oil, being careful not to overcrowd the pot. Fry for 3-5 minutes, turning occasionally, until they are golden brown and crispy on all sides.
Step 5: Remove the potato puffs with a slotted spoon and transfer them to a plate lined with paper towels to drain excess oil. Season lightly with a pinch of salt immediately after frying.
Step 6: Serve hot with your favorite dipping sauce, such as sour cream, ranch, or ketchup. Enjoy your irresistible Cheesy Mashed Potato Puffs!