Place the potatoes in a large pot of lightly salted water and bring to a boil over high heat. Reduce the heat to medium. Cook, uncovered, until the potatoes are tender when pierced with a knife, 25 to 35 minutes. Drain and rinse under cold water. Cool completely. Cover and refrigerate until chilled, at least 4 hours and preferably overnight.
Position a rack in the center of the oven and preheat to 400 degrees F. In a very large (12-inch) nonstick skillet (if the skillet handle isn't heatproof, wrap it in a double thickness of aluminum foil), heat the butter over medium heat. Add the leeks and cook until tender, stirring often, about 4 minutes. In a small bowl, mix the salt and pepper. Season the leeks with 1/4 teaspoon of the salt-and-pepper mixture. Transfer to a small bowl and set aside. (The leeks can be prepared up to 1 day ahead, cooled, covered and refrigerated.)
Peel the potatoes and shred on the largest holds of a box grater. In the skillet, heat 2 tablespoons of the goose fat over medium-high heat until very hot but not smoking. Spread half of the potatoes in the skillet in a layer. Season with 1/2 teaspoon of the salt-and-pepper mixture. Spread with the leeks, leaving a 1-inch border around the edges. Top with the remaining potatoes, and season with the remaining 1/2 teaspoon salt and pepper.
Cook until the edges of the pancake are golden brown, about 5 minutes. Hold a flat round skillet top, plate, or pizza pan on top of the skillet. Inver the skillet and the skillet top together so the pancake falls, upside down, onto the skillet top. (If this seems too heavy to handle, slide the pancake carefully out of the skillet onto a plate. Place a second plate on top and invert.) Return the skillet to the stove and heat the remaining tablespoons goose fat until very hot. Slide the pancake back into the skillet, browned side up. Cook until the underside is browned, 5 to 7 minutes.
Bake for 10 minutes. Turn the pancake as before, and bake it until crisp, about 10 more minutes.
Transfer to a warm round platter and sprinkle with the chives, if desired. Serve hot, cut into wedges.
Recipe courtesy of Rick Rodgers, Christmas 101, Random House, 1999