You might not think of nori as a French ingredient, but the seaweed can be found on rocks along the coast and has been cultivated on seaweed farms around Brittany for decades.
2 pounds russet or Yukon gold potatoes, peeled
4 tablespoons (1/2 stick) butter, plus more for garnish
½ cup buttermilk
3 tablespoons nori flakes or chopped nori
Cook the whole potatoes in a large pot of boiling, salted water 25 to 30 minutes, or until soft. Drain, and reserve cooking water.
Return the potatoes to the pot, and mash with the butter and ¼ cup of the cooking water until smooth. (Don’t overmash.) Stir in the buttermilk, and the nori, and season with salt and pepper. Taste, and add a little more buttermilk if the potatoes seem too dry. Cover, and let stand 5 minutes before serving so the flavors can develop. Serve with an extra dollop of butter on top. Serves 6