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1
Place the potatoes in a large pot of lightly salted water and bring to a boil over high heat.
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2
Reduce the heat to medium.
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3
Cook, uncovered, until the potatoes are tender when pierced with a knife, 25 to 35 minutes.
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4
Drain and rinse under cold water.
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5
Cool completely.
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6
Cover and refrigerate until chilled, at least 4 hours and preferably overnight.
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7
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.
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8
Add the leeks and cook until tender, stirring often, about 4 minutes.
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9
In a small bowl, mix the salt and pepper.
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10
Season the leeks with 1/4 teaspoon of the salt-and-pepper mixture.
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11
Transfer to a small bowl and set aside.
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12
(The leeks can be prepared up to 1 day ahead, cooled, covered and refrigerated.)
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13
Peel the potatoes and shred on the largest holds of a box grater.
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14
In the skillet, heat 2 tablespoons of the goose fat over medium-high heat until very hot but not smoking.
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15
Spread half of the potatoes in the skillet in a layer.
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16
Season with 1/2 teaspoon of the salt-and-pepper mixture.
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17
Spread with the leeks, leaving a 1-inch border around the edges.
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18
Top with the remaining potatoes, and season with the remaining 1/2 teaspoon salt and pepper.
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19
Cook until the edges of the pancake are golden brown, about 5 minutes.
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20
Hold a flat round skillet top, plate, or pizza pan on top of the skillet.
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21
Inver the skillet and the skillet top together so the pancake falls, upside down, onto the skillet top.
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22
(If this seems too heavy to handle, slide the pancake carefully out of the skillet onto a plate.
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23
Place a second plate on top and invert.)
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24
Return the skillet to the stove and heat the remaining tablespoons goose fat until very hot.
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25
Slide the pancake back into the skillet, browned side up.
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26
Cook until the underside is browned, 5 to 7 minutes.
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27
Bake for 10 minutes.
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28
Turn the pancake as before, and bake it until crisp, about 10 more minutes.
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29
Transfer to a warm round platter and sprinkle with the chives, if desired.
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30
Serve hot, cut into wedges.
