Method --
Preheat oven to 375 degrees F. Butter 2 large ramekins. Cover the bottom of each ramekin with potato slices, brush with softened truffle butter and add another layer of potato. Repeat until all potatoes are used. Weigh down each potato cake with a smaller ramekin, then bake in the oven until egdes are crisp and the center is cooked through, about 10 minutes. Using an offset spatula, remove potato cakes to paper toweling.
In a small bowl, combine the demi-glace, truffle juice and chopped truffles, set aside.
Season filets with salt & pepper. In a large skillet over medium heat. Melt the duck fat in a large skillet over medium-high flame. Sear the filets until desired doneness, about 5 minutes each side for medium-rare. Remove to a cutting board to rest, tent with foil to keep warm.
Discard all fat from the skillet. While the pan is still hot, add the Madeira, scraping up all the beefy bits on the bottom. Add the demi-glace mixture, cook until reduced, then remove from heat and stir in the truffle butter. Taste for seasoning and add salt & pepper, if necessary. Keep warm.
Heat a small, dry skillet over high flame. When hot, sear the foie gras slices until golden brown, about 60 seconds on each side. Remove to paper toweling.
On each of two plates, place the potato cake in the center and top with the filet mignon then foie gras. Spoon the sauce over and around. Top with freshly shaved truffle.
