Scatter the coriander seeds over the bottom of a small skillet. Toast over moderately low heat, shaking the pan frequently, for 3-4 minutes or until you get just a whiff of thir fragrance. Place a sheet of paper towel on you work surface an pour the seeds onto it. Use of the side of a chef's knife or the flat bottom of a heavy glass to crush the seeds. Careully tilt the seeds in to a small bowl; set aside.
Place the potatoes in a large pot with enough cold water to cover them by 2 inches. Add kosher salt to taste (about 1 tsp per quart). Bring to a boil. Reduce heat to moderate, and cook until the potates are tender whn pierced with the tip of a knife, 25-30 minutes.
Drain the potaotes and keep warm. Just before serving, transfer them to a shallow casserole. Using a parin knife, peel the potatoes, if desired. With a fork of poato masher, crush the potaotes gently to just split them open. Sprinkle lightly with sea salt. Stir the creme fraiche until smooth and spoon it over the potatoes. Cut the butter into small pieces and sprinkle it over the potatoes. Scatter the coriander over the top and sprinkle with more sea salt and freshly ground chives, if desired.