Wash the potatoes. Wrap them in foil. Bake in the oven at 400 degrees F for about an hour, or until tender. Peel the potatoes and mash. This can be done a day or two ahead of making the soup.
In a large soup pot, melt the butter over medium heat. Add the flour and cook for a couple of minutes, stirring often.
Whisk in the milk and simmer until starting to thicken, about 8 minutes
Stir in the mashed potatoes, salt and pepper. Simmer for about 5 minutes.
Reduce the heat to low and stir in the chives, one cup of the cheddar cheese, and the sour cream. Heat through.
Serve the soup topped with more cheese and some crumbled bacon.