Preheat oven to 400 degrees F (200 degrees C). Grease a 9x13-inch casserole dish.
Poke a few holes into each potato using a toothpick.
Bake potatoes in the preheated oven until fully cooked, about 1 hour. Cool for about 15 minutes.
Place the bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain the bacon slices on paper towels and crumble.
Cut a thin slice from one side of each potato; carefully scoop out the flesh and transfer to a bowl. Discard skins. Mix 3/4 cup Cheddar cheese, sour cream, milk, butter, chives, salt, black pepper, and garlic powder with potatoes. Spread potato mixture into the prepared casserole dish; top with 3/4 cup Cheddar cheese and crumbled bacon.
Bake in the preheated oven until just bubbling, 10 to 15 minutes. Cool for at least 5 minutes before serving.