Remove stems, and cut a 1/4- inch slice from the top of each tomato. Scoop out pulp, leaving shells intact; reserve pulp for other uses. Invert tomato shells onto paper towels to drain; set aside.
Sauté mushrooms in butter in a large skillet until tender. Remove from heat, and gradually stir in sour cream, salt, and pepper.
Spoon mixture into prepared tomato shells; sprinkle bacon on top of each. Place tomatoes in a lightly greased 15- x 10- x 1-inch jellyroll pan. Bake at 350° for 15 to 20 minutes. Garnish with chives; serve warm.