Preheat oven to 425 degrees. Wash potatoes and poke 2 or 3 vent holes in each with a fork. Bake potatoes in oven until done, about 1 hour. (Poke with a fork and if fork goes in easily they are done). Remove potatoes from oven and allow to cool for 10 minutes. Slice potatoes in 1/2 lengthwise and scoop pulp into mixing bowl being careful not to break skin. Mash potatoes with butter, cheese, milk, and chives. Taste-test and adjust seasoning if necessary with salt and pepper. Spoon mashed potatoes back into skins and place in a 9 by 13-inch dish. Return to oven and heat thoroughly.