Prepare Baked Potatoes; cool 10 minutes. Meanwhile, in a 10-inch skillet melt butter over medium heat. Add mushrooms and garlic; cook 5 to 6 minutes or until mushrooms are tender and browned, stirring occasionally.
Cut a lengthwise slice from top of baked potatoes; discard skin from slices and place pulp in a bowl. Scoop pulp from potatoes, leaving 1/2-inch shells and adding pulp to bowl. Mash pulp. Stir in sour cream, salt, and pepper until smooth. (If needed, stir in 1 to 2 Tbsp. milk to reach desired consistency.) Stir in mushroom mixture, 1/2 cup of the cheese, the bacon, and, if desired, chives. Stuff potato shells with mashed potato mixture. Place in a 2-qt baking dish.
Bake in the 425 degrees F oven 20 to 25 minutes or until light brown. Sprinkle with remaining 1/4 cup cheese. Bake 2 to 3 minutes more or until cheese is melted.
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 potatoes in the preheated oven 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 your hand. Using a knife, cut an X in top of each potato. Press in and up on the ends of each potato.