Cook potatoes, chicken stock and salt in a covered 4- to 5-quart slow cooker until potatoes are tender, on high for 2 1/2 to 3 hours or on low for 4 to 5 hours. Drain and return potatoes to slow cooker. Add sour cream and mash to desired consistency. Fold in chives and warm milk (add more milk to loosen if desired). Season with salt and pepper. Serve warm, topped with chives.