Put the potatoes in a large saucepan and cover with water by 2 inches. Add 2 tablespoons of kosher salt, bring to a boil and cook until fork tender, about 25 minutes.
While the potatoes are cooking, heat the milk in a small saucepan over high heat, bring to a boil, cover and remove from the heat. Keep warm until the potatoes finish cooking.
Drain the potatoes in a colander and return them to the saucepan over low heat. Stir the potatoes for about a minute in the saucepan to evaporate any additional water.
Put the potatoes into a ricer or food mill set over a large bowl. Stir in the butter then 1 1/2 cups to all of the warm milk until creamy. Stir in creme fraiche, mascarpone or pesto, if desired. Season with salt and pepper.