Heat the oven to 425°F. Pierce potatoes several times, rub with 1 teaspoon oil and bake directly on the rack for 60 to 70 minutes until completely tender. Cool for 5 minutes.
Meanwhile, cook bacon in a skillet over medium heat for 8 to 10 minutes until crisp. Drain and crumble.
Split hot potatoes and scoop flesh into a warm bowl. Cut the skins from 2 potato halves into short strips, toss with remaining oil and bake on a rimmed sheet for 8 to 10 minutes until crisp; reserve other skins for another use.
Mash the warm flesh with butter, then fold in warm milk gradually, sour cream, cheddar, salt, pepper and half the bacon until spoonable and the cheese softens. Avoid a mixer or food processor.
Transfer to a warm serving bowl and add remaining bacon, the measured scallions and crisp skin pieces just before serving.