This loaded mashed potato casserole can be prepped up to 2 days ahead. It can be prepped earlier and frozen as well.You can use leftover mashed potatoes or fresh cooked potatoes.
Mix together potatoes, milk, butter, cream cheese, sour cream, half of the shredded cheese, chives, garlic powder, salt, parsley and pepper together in a mixing bowl.
It is best to use a mixer (hand or stand mixer) to mix the potatoes with everything, but it's not required. You can use a masher, but a mixer will get it smooth with no large lumps.