Position 2 racks in the center of the oven, and preheat to 375 degrees F. Thoroughly wash and dry the potatoes, then prick each all over 8 to 10 times with a fork. Rub the butter evenly on the potatoes, and sprinkle liberally with salt and pepper. Place the potatoes on a baking sheet with at least 1 inch of space between them. Put the bacon in a single layer on a rimmed baking sheet lined with parchment paper.
Place both baking sheets in the oven, the bacon below the potatoes. Cook the bacon, flipping halfway through, until golden brown and crispy, 18 to 20 minutes; transfer to a paper-towel-lined plate, let cool and then crumble. Bake the potatoes, flipping halfway through, until the skin is crispy and a small knife inserted in the center goes in and out without any resistance, 45 to 50 minutes. Transfer the potatoes to a cutting board, and let cool completely, about 2 hours; quarter each cooled potato lengthwise, then cut into 1-inch pieces.
Whisk together the sour cream, mayonnaise, 2 tablespoons water, the apple cider vinegar and 1 teaspoon each salt and pepper in a large bowl. Add the potatoes, bacon, Cheddar and white parts of the scallions, and gently stir to combine. Season with additional salt and pepper if needed.
Serve immediately, or refrigerate until ready to serve. Garnish with the scallion greens right before serving.
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