Peel the potatoes and cut into pieces. Place in a large saucepan and cover with cold water seasoned with salt to taste. Cover pan and bring to a boil. Uncover and simmer for 20 to 25 minutes, or until the potatoes are fork tender. Drain, reserving the liquid, and return to the hot pot. Heat the milk. On lowest speed of the portable mixer, start mashing the potatoes. When they are broken into small pieces, gradually add the hot milk and whip until smooth. Add the cheese and whip until melted. Add some of the reserved cooking water, as needed, for a fluffy texture. Season with salt to taste and the pepper. Serve warm.