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.