Cut a lengthwise slice off the top of each baked potato; discard skin from slices and place pulp in a bowl. Scoop pulp out of potatoes; add 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 pepper; beat until smooth. If necessary, stir in milk to reach desired consistency. Season to taste with salt and additional pepper. Stir in 1/2 cup of the cheddar cheese and chives, if desired. Divide the mashed potato mixture evenly among the potato shells. Place in a 2-quart baking dish.
Bake, uncovered, in a 425 F oven for 20-25 minutes or until light brown. Sprinkle with remaining cheese. Bake for 2-3 minutes more or until cheese melts.