Scatter the coriander seeds over the bottom of a small skillet. Toast over moderately low heat, shaking the pan frequently, for 3 to 4 minutes or until you get just a whiff of their fragrance. Place a sheet of paper towel on your work surface and pour the seeds onto it. Use the side of a chef's knife or the flat bottom of a heavy glass to crush the seeds. Carefully tilt the seeds into 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 teaspoon per quart). Bring to a boil, reduce heat to moderate, and cook until the potatoes are tender when pierced with the tip of a knife, 25 to 30 minutes. Drain the potatoes and return to the pot. Cover and keep warm until ready to serve. Just before serving, transfer them to a shallow casserole. Using a paring knife, peel the potatoes, if desired. With a fork or potato masher, crush the potatoes gently to just split them open. Sprinkle lightly with sea salt. Stir the crème fraîche 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 pepper. Top with the chives, if desired, before serving.