Preheat the oven to 400° F.

Generously butter 6 (8-ounce) ramekins with 1 tablespoon of butter. Add grated Parmesan to ramekins, turning to coat the bottom and sides of each one with the cheese. Place ramekins on a large rimmed baking sheet. Warm milk in a microwave or on the stove top.

Add remaining 3 tablespoons butter to a medium saucepan over medium heat. When the butter melts, whisk in flour, creating a roux. Cook the roux for about 1 minute; it will bubble, take on a golden hue and smell toasty. Pour in the warm milk, whisking until the mixture combines and there are no lumps to create a béchamel sauce.

Simmer the béchamel for a few minutes until it thickens. Remove the pan from heat and whisk in salt, herbs and dry mustard. Set aside to cool while you whip the egg whites.

In the bowl of a stand mixer or using a hand mixer, whisk egg whites and cream of tartar, if using, until stiff peaks form, about 3 minutes.

Transfer béchamel to a large bowl and whisk in egg yolks, 1 or 2 at a time. Transfer 1 large scoop of whipped egg whites to the bowl with the béchamel and whisk in gently to lighten the mixture. Add 1/3 of the remaining whites and using a large spatula, gently fold them into the base. Sprinkle the grated cheese over the mixture and gently fold in the remaining whites in 2 additions. The whites should be incorporated but not deflated.

Evenly divide the batter among the prepared ramekins. Run your thumb around the top edges of each ramekin to remove any excess batter — this will help with an even rise. Transfer the baking sheet with the soufflés to the preheated oven and reduce the temperature to 375° F.

Bake until the soufflés have puffed and risen, are golden brown and set in the center, about 18 minutes. Test for doneness by tapping the side of a ramekin; the center should jiggle slightly but not be wet. If not quite set, quickly return to the oven and bake for 2 minutes more. Serve immediately.