1. Peel the potatoes and cut them into large chunks. Place them in a medium-sized saucepan and cover them by 1-inch (2.5cm) water. Add 1/2 teaspoon coarse sea salt. Bring the water to a boil over medium high heat and cook until the potaotes are tender, for about 20 minutes.
2. Drain the potatoes, reserving the cooking liquid. Return the potatoes to the saucepan with the butter and the creme fraîche, and mash the potatoes, using a potato masher or a fork. You want the potatoes smooth but not elastic, the way they can become if you mash them in a machine. Add more butter and cream if you like or, if they are too thick you can also add potato cooking water. Season the puree to taste with salt and pepper , and serve.