In a bowl, combine the cold mashed potatoes, shredded cheddar, chopped chives, garlic powder, salt, and pepper. Mix until evenly combined.
Scoop about 2 teaspoons of the mixture and roll it into a 1-inch ball. Repeat. If the mixture is too soft, chill the balls for 10-20 minutes to help them hold their shape.
Place flour, beaten egg, and panko crumbs in separate shallow bowls. Dredge each ball first in the flour, then in the egg, and finally in the panko.
Heat 2-3 inches of oil in a deep pot to 375°F. Fry the balls in small batches for 2-3 minutes until all sides are golden and crispy.
Remove the balls with a slotted spoon and drain on paper towels before serving. Garnish with parsley and serve with sour cream or your preferred dipping sauce.