Place the potatoes in a large pot, cover with water and bring to a boil over high heat. When the water begins to boil, reduce the heat to medium-high. Continue to boil until potatoes are cooked through, about 15-20 minutes. Drain the potatoes and then put them back into the pot. Add the milk, butter, sour cream and salt to the pot and using a potato masher, mash the potatoes until no large lumps remain. Season to taste with salt and pepper. Serve.