Arrange a rack in the middle of the oven and heat the oven to 400°F.
Place 4 large russet potatoes directly on the oven rack. Bake until cooked through and easily pierced to the center with a knife, 1 hour to 1 hour 15 minutes. Meanwhile, coat an 8x8-inch square or other 2-quart baking dish with cooking spray. Place 4 tablespoons unsalted butter and 4 ounces cream cheese in a large bowl and let sit at room temperature until softened. Cook the bacon.
Working in batches if needed, cook 1 pound bacon in a large skillet over medium heat until crispy, about 8 minutes. (Alternatively, you can cook the bacon on a large rimmed baking sheet in the oven while the potatoes cook. It will take 12 to 14 minutes, depending on the thickness of the bacon.) Let drain on a paper towel-lined plate, then crumble into small pieces.
Remove the baked potatoes from the oven. When cool enough to handle, cut in half lengthwise. Scoop the flesh out with a spoon into the bowl of butter and cream cheese. Add 2/3 cup sour cream and 1/2 cup whole milk and mash with a potato masher until the butter and cream cheese are melted and everything is combined. If the mixture is very thick, mix in more milk as needed.
Finely chop 1 large bunch fresh chives until you have 6 tablespoons. If needed, shred 8 ounces sharp cheddar cheese on the large holes of a box grater (about 2 cups).
Add 1/4 cup of the chives, 1 1/4 cups of the cheese, all but 1/2 cup of the bacon, and 1 teaspoon seasoned salt to the mashed potatoes and stir to combine. Transfer to the baking dish and spread into an even lyer. Sprinkle with the remaining 3/4 cup cheese.
Bake heated through and the cheese is melted, 20 to 25 minutes. Sprinkle with the remaining bacon and chives.
