In a soup pot, combine the potatoes, 2 teaspoons salt and enough cold water to cover. Bring to a boil and cook until the potatoes are fork-tender, about 20 minutes. Meanwhile, in a small saucepan, warm the milk over medium heat. Drain the potatoes and return to the pot. Pour in the warmed milk and mash to combine. Add the butter and mash until incorporated. Season with salt.