Preparation:
Cook the bacon pieces over medium heat in a medium skillet until crisp. Transfer bacon to paper toweling to drain and set aside. Peel, rinse, and cut the potatoes into thirds. Place them in a large pot with water to cover, add 2 teaspoons salt, and bring to a boil. Reduce heat to a simmer and cook until the potatoes are very soft -- about 45 minutes. Drain the potatoes, discarding the water, and return them to the pot. Mash them with a potato masher until smooth. Add the sour cream and butter and stir until melted. Add the milk, pepper, and remaining salt and bring the soup back to a simmer. Divide among 8 bowls and serve the soup hot, garnished with scallions, cheese, and bacon.