Preheat oven to 400°F (200°C). Scrub potatoes clean, pat dry, poke several times with a fork, rub all over with olive oil, and sprinkle generously with salt.
Place potatoes directly on the oven rack or on a foil-lined baking sheet. Bake for 50 to 60 minutes, until a fork slides easily into the center.
Remove potatoes from oven and let rest for about 10 minutes. Reduce oven temperature to 375°F (190°C) for the second bake.
Slice each potato in half lengthwise. Scoop the flesh into a large mixing bowl, leaving a thin quarter-inch border inside each skin so the shells hold their shape. Set hollowed skins aside on the baking sheet.
Add softened butter, sour cream, and milk to the bowl of potato flesh. Mash and beat until smooth and fluffy using a hand mixer or potato masher.
Fold in 1 cup of shredded cheddar, crumbled bacon, chopped chives, and garlic powder. Season with salt and black pepper to taste.
Spoon filling back into each hollowed potato skin, mounding it slightly on top. Divide evenly across all 8 potato halves.
Sprinkle the reserved 1/2 cup of cheddar cheese evenly over the tops of the filled potatoes.
Return filled potato halves to the oven at 375°F (190°C) and bake for 15 to 20 minutes, until cheese is melted and lightly golden.
For an extra golden top, switch oven to broil for the last 1 to 2 minutes, watching closely so the cheese doesn't burn.
Remove potatoes from oven and let cool for a couple of minutes. Sprinkle remaining chopped chives over the top and serve warm with extra sour cream on the side if desired.