The challenge of soufflés lies not in technique but in timing, because once a soufflé is pulled from a hot oven, it begins to deflate. One easy work-around is to bake the soufflés twice, a trick I learned from Anne Willan at La Varenne; it liberates the cook from the high-stakes moment of pulling a soufflé from the oven and serving it before it deflates.
The individual soufflés are baked once, turned out of the ramekins into a baking dish, coated with cream sauce and cheese, then baked—up to twenty-four hours later!—a second time. They puff up again, as forgiving as can be. Magic!
Preheat the oven to 425°F and generously grease six 8-ounce ramekins. In a medium saucepan over medium-low heat, melt 1 tablespoon of the butter. Add the leeks and cook, stirring, until soft but not brown, 4 to 5 minutes. Transfer to a bowl and set aside.
Melt the remaining 4 tablespoons of butter over medium heat in the same saucepan that you used for the leeks. When the butter stops foaming, whisk in the flour and cook, whisking, for 1 minute. Whisk in the milk and cook, whisking, until the mixture boils and thickens. Stir in the salt, nutmeg, 1 1/4 cups of the cheese, and the leeks. Transfer a third of the mixture to a bowl, whisk in the cream, and set aside. Whisk the egg yolks into the remaining two-thirds, then transfer the mixture to a large bowl.
In the bowl of an electric mixer fitted with a whisk attachment (or in a large bowl with a handheld mixer), beat the egg whites with a pinch of salt until they hold stiff peaks. Stir a third of the whites into the yolk mixture to lighten it, then fold in the remaining two-thirds until no streaks of white remain. Divide the mixture among the greased ramekins, smooth the tops, then run the tip of your finger around the inner edge of each ramekin (this will help the soufflé rise higher and straighter). Arrange the ramekins in a baking dish and pour enough hot water into the baking dish to come half an inch up the side of the ramekins.
Transfer to the oven and bake until puffed, deep golden brown, and set within, about 25 minutes. Remove from the oven, remove the ramekins from the water bath, and let the soufflés cool (they will deflate).
Run a knife around the inner edge of each ramekin, then turn the soufflés out into a gratin dish and pour the reserved cream mixture over and around the soufflés. Top each with some of the reserved Parmigiano. At this point, the soufflés can be covered with plastic wrap and refrigerated for up to 24 hours (I’m telling you, this recipe is magic).
When you’re ready to bake the soufflés a second time, preheat the oven to 425°F. Bake until the soufflés are puffed and browned (they will puff as much as—if not more than—the first time they were baked), about 10 to 15 minutes. Serve immediately.