Line a baking sheet with foil. Preheat broiler, placing oven rack directly beneath heat source. Pierce potatoes with a fork all over and cook on high in microwave until tender, about 20 minutes, turning once. While potatoes are cooking, sauté bacon in a small, heavy-bottom skillet over medium-high heat until crisp and brown, about 4 minutes per side. Transfer bacon to paper towels to drain. When cool enough to handle, crumble bacon and set aside.
Put butter and milk in a microwave-safe bowl and microwave for 2 minutes. With hands covered to protect from heat, cut potatoes in half lengthwise and scoop out flesh with a spoon. Place potato flesh in a medium bowl and break up with a fork; set aside potato skins. Add milk mixture, salt, pepper, 3/4 cup cheese and scallions to bowl with potato flesh and mix well. Stir in bacon. Divide potato mixture among potato skins.
Place potatoes on prepared baking sheet and sprinkle with remaining 1/4 cup cheese. Broil potatoes until tops are crisp and bubbling and mixture is heated through, about 4 minutes.