Place potatoes in a Dutch oven; add cool water to cover by 2 inches. Bring to a boil over high heat; reduce heat to medium, and simmer until tender, 11 to 14 minutes. Remove from heat; drain. Return potatoes to pan; mash with a potato masher to desired consistency. Weigh or lightly spoon flour into a dry measuring cup; level with a knife. Whisk together flour, milk, and stock in a medium bowl. Stir milk mixture into potatoes; cook over medium, stirring often, until thickened and bubbly, about 4 minutes. Remove from heat; stir in sour cream, salt, and pepper. Ladle soup into 6 bowls; top with cheese, bacon, and chives.