Preheat oven to 425. Prick each potato with fork 3-4 times then place on sheet tray. Roast for 55-70 minutes until the potatoes are tender when pressed. Remove the potatoes from the oven and lower the temperature to 375 degrees.
Meanwhile, working in two batches, cook the bacon in a large skillet over medium heat until golden and crispy, 6-8 minutes. Let the cooked bacon drain on a paper towel lined plate. Once cool, crumble or chop into small pieces.
Place the melted butter, cream cheese, warm milk, and sour cream in a large bowl. Once the potatoes are cool enough to handle, cut them in half lengthwise, and scrape the flesh out of the skins and into the bowl. Tear or chop up three of the potato skins and add them to the bowl. Mash everything together with a potato masher. Fold in half of the bacon, half of the cheddar cheese, parmesan, and half of the chives.
Grease a 3-quart casserole dish with butter. Transfer the potato mixture to the baking dish. Cover and bake for 35 minutes until the edges are bubbly. Top with the remaining bacon and cheese. Bake for 15-20 more minutes until warmed through and lightly golden on top. Let rest 10 minutes, then top with the remaining chives and serve.