Peel and chop 3 pounds of russet potatoes into 1-inch pieces. Place in a large pot and add enough water to cover the potatoes by 2 inches. Bring to a boil over high heat, about 15 minutes. Reduce the heat to maintain a simmer and cook until the potatoes are tender and can be easily poked with a paring knife, 7 to 10 minutes. Meanwhile, warm the milk mixture and prepare the mix-ins.
Place 6 slices of bacon in a large skillet in a single layer. Cook over medium heat, flipping halfway through, until crisp, about 8 minutes total. Transfer to a paper towel-lined plate to drain, then crumble into small pieces.
Place 1/2 cup whole milk, 6 tablespoons unsalted butter, 1 teaspoon kosher salt, and 1/4 teaspoon black pepper in a medium saucepan. Warm over medium heat, stirring constantly, until the butter is melted, 2 to 5 minutes. Remove from the heat.
Grate 8 ounces sharp cheddar cheese on the large holes of a box grater (about 2 cups). Finely chop 1 large bunch fresh chives until you have 1/4 cup.
When the potatoes are ready, drain through a heatproof colander. Place the colander over the pot. When cool enough to handle but still warm, return to the pot and mash with a potato masher until mostly smooth (a few lumps are fine). (Alternatively, pass the potatoes through a ricer or food mill back into the pot.)
Add the warm milk mixture, bacon, cheese, 1 cup sour cream, and all but 1 tablespoon of the chives. Stir to combine (the cheese won’t be completely melted). Transfer to a serving bowl. Garnish with the reserved chives and serve.
