Place an oven rack in middle position and heat oven to 350 degrees. Grease the bottom and sides of an 8-1/2 by 4-1/2-inch glass loaf pan, a 1-1/2 quart soufflé dish, or a 1-1/2 quart ovenproof pot. Sprinkle 2 tablespoons Parmesan into the pan and shake to coat evenly.

Melt 3 tablespoons butter in a medium saucepan or skillet, over medium heat. Add shallot and cook until softened, about 2 minutes. Stir in flour and cook until golden, about 1 – 2 minutes. Slowly whisk in milk. Bring to a simmer and cook, whisking until thickened and smooth, about 1 -2 minutes. Remove from heat, whisk in cheese, salt, pepper, mustard, nutmeg and vanilla. Transfer mixture to a large bowl. Whisk in egg yolks until completely incorporated and set aside.

In a separate bowl, beat egg whites and cream of tartar (with an electric mixer at medium-high speed or whisk by hand) until stiff peaks form.

Working with a quarter of the whipped egg whites at a time, gently fold them into the yolk mixture until almost no white streaks remain. Pour mixture into prepared pan and sprinkle with remaining 2 tablespoons Parmesan. Bake until top is nicely browned and center jiggles slightly, 25 – 30 minutes. Bring the soufflé straight from the oven to the table. Serves 4