Clean potatoes and wrap in tin foil. Bake at 400 degrees for 1 hour or until tender when poked with a fork. Peel and chop into 1/2" cubes. Set aside.
Cook bacon in a skillet over medium heat until fat is rendered and bacon is crisp. Remove from the pan and drain on a paper towel. Crumble and set aside.
In a dutch oven or deep soup pot, melt 2/3 cup butter over medium heat. Once butter is melted, slowly whisk in 2/3 cup flour. Whisk until smooth and flour is cooked (about 1-2 minutes.) Gradually add six cups of milk to the butter and flour mixture, whisking until smooth. Cook over medium heat until thickened.
Add potatoes and 6 sprigs of chopped chives. Turn burner to medium-high heat and cook for 10-15 minutes, stirring frequently.
Reduce heat to a simmer and stir in 1/2 of bacon, 1 cup cheese, and 8 ounces of sour cream. Add more milk, if the soup is too thick. Cook for another 5-10 minutes. Salt and pepper to taste.
Serve in deep bowls. Top each serving with reserved shredded cheese, crumbled bacon, and chives.