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Melt the butter in a large skillet. Add the leeks and cook over moderately low heat, stirring occasionally, until softened but not browned, about 10 minutes. Season with salt and pepper and let cool to room temperature.
Butter an 8-by-10-inch baking dish and line it with the tortillas. In a large bowl, whisk the eggs with the egg yolks. Add the cream and milk and whisk to blend. Stir in the leeks, corn, 2 teaspoons salt and 1/4 teaspoon pepper. Pour the custard over the tortillas. Cover and refrigerate overnight. Bring to room temperature before baking.
Preheat the oven to 325°. Put the baking dish in a roasting pan and add enough hot tap water to reach halfway up the side of the dish. Bake the corn pudding in the water bath for about 1 hour, or until just set. Let stand for about 10 minutes before serving.
The unbaked pudding can be refrigerated for up to 2 days.