Using a 6-quart slow cooker, add potatoes, butter, salt and pepper. Stir and add 1 cup of water/stock. Cover and cook on high for 4½-5 hours, or until potatoes are tender.

Add sour cream and warm milk (start with ⅓ cup).

Transfer potatoes to a large mixing bowl. Using an electric mixer, blend potatoes on high speed for 4-5 minutes or until smooth and creamy. (Add several more Tbsp of additional warm milk, as needed.)

Garnish with chives before serving.