Prepare potatoes as directed below; let stand about 10 minutes. Cut a lengthwise slice from the top of each baked potato; discard skin from slice and place pulp in a bowl. Scoop pulp out of each potato. Add the pulp to the bowl.
Mash the potato pulp with a potato masher or an electric mixer on low speed. Add sour cream, garlic salt, and black pepper; beat until smooth. (If necessary, stir in 1 to 2 tablespoons milk to reach desired consistency.) Season to taste with salt and additional black pepper. Stir in 1/2 cup of the cheddar cheese and, if desired, chives. Spoon the mashed potato mixture into the potato shells. Place in a 2-quart baking dish.
Bake, uncovered, in a 425 degrees F oven for 20 to 25 minutes or until light brown. Sprinkle with remaining cheese. Bake for 2 to 3 minutes more or until cheese melts.
Preheat oven to 425 degrees F. Scrub potatoes thoroughly with a brush; pat dry. Prick potatoes with a fork. (If desired, for soft skins, rub potatoes with shortening or wrap each potato in foil.)
Bake for 40 to 60 minutes (or in a 350 degrees F oven for 70 to 80 minutes) or until tender. To serve, roll each potato gently under a towel to break apart the insides. Using a knife, cut an X in the top of each potato. Press in and up on the ends of each potato to open.