Toss the potatoes, butter, 1 tablespoon salt and 1/2 teaspoon pepper together in a 6-quart slow cooker. Add 1 cup of water, then cover and cook on high until the potatoes are tender, about 4 hours. (The potatoes might have browned lightly in spots but that is okay.)
Remove the lid and add the sour cream and 1 1/2 cups warm milk to the potatoes. Use a potato masher to smash the potatoes until they are mostly smooth. If the potatoes need to be thinned out, stir in the remaining 1/2 cup warm milk. Season to taste with additional salt and pepper. Garnish with chives.
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