Preheat the oven to 350°F. Butter an 8 x 8-inch, 2-quart casserole. Place a layer of yams slightly overlapping each other in the bottom of the casserole. Follow with a layer of potato. Sprinkle with salt and pepper. Repeat the process, finishing with the yams. Pour the heavy cream over the yams and potatoes. Cover the casserole with foil. Bake for 1 hour. Remove the foil from the casserole and bake until the yams are golden brown, about 20 more minutes. Remove from the oven and let sit for 10 minutes. Cut the gratin into squares and remove the squares from the casserole with a metal spatula. Serve immediately. Recipe from Maine Classics by Mark Gaier and Clark Frasier/Running Press, 2011.