4 tablespoons unsalted butter, plus softened butter for brushing
1/4 cup freshly grated Parmigiano-Reggiano cheese
1/4 cup plus 2 tablespoons all-purpose flour
1 1/2 cups milk
3/4 teaspoon salt
1/4 teaspoon cayenne
6 ounces diced ham
2 sliced leek whites sautéed in butter
6 large eggs, separated
5 ounces shredded aged Gouda
1/2 teaspoon cream of tartar
Preheat the oven to 400° and brush eight 1-cup ramekins with butter. Lightly coat the ramekins with 2 tablespoons of the Parmigiano-Reggiano and set them on a sturdy baking sheet.
In a medium saucepan, melt the 4 tablespoons of butter. Whisk in the flour and cook over moderate heat for 1 minute. Whisk in the milk and cook over moderately low heat until smooth and very thick, about 2 minutes. Stir in the salt and cayenne. Off the heat, whisk in the egg yolks. Let cool slightly. Transfer to a large bowl and stir in the Gouda, ham and leeks.
In a large bowl, using an electric mixer, beat the egg whites with the cream of tartar at medium-high speed until frothy. Increase the speed to high and beat until firm peaks form. With a rubber spatula, fold the egg whites into the soufflé base until no streaks of white remain.
Spoon the soufflé mixture into the ramekins, filling them to 1/2-inch below the rim. Run your thumb inside the rim of each ramekin to help the soufflés rise evenly. Sprinkle the remaining Parmigiano-Reggiano on top and bake in the bottom third of the oven until the soufflés are puffed and golden brown, about 20 minutes. Alternatively, bake in an 8-cup soufflé at 375° for 40 minutes. Serve immediately.
The recipe can be prepared through Step 2 and refrigerated overnight. The soufflés can be assembled up to 1 hour ahead. The cooked soufflés can be reheated in the oven.