Preheat the oven to 400°F (200°C). Position a rack in the lower third of the oven. Butter four 10-ounce (300 ml) ramekins thoroughly, then dust the insides with the grated Parmesan—this gives the soufflés something to ‘climb’ on and encourages even rising. Place the prepared ramekins on a baking sheet. Preheating ensures immediate oven spring when the soufflés go in.

Separate the eggs while they are cold (cold eggs separate more cleanly): place yolks in one bowl and whites in a large, dry mixing bowl. Bring the milk to warm (not boiling) in a small saucepan — you want it steaming with small bubbles at the edge; heating the milk keeps the béchamel smooth and helps the flour cook evenly when added.

Make a classic béchamel: melt 3 tbsp butter in a medium saucepan over medium heat. Add the 3 tbsp flour and whisk constantly for 1–2 minutes to cook the raw flour taste but do not let it brown — this creates a pale roux that will thicken the milk without adding color or a toasted flavor.

Gradually whisk the warm milk into the roux in a steady stream, whisking constantly to avoid lumps. Bring to a gentle simmer and cook for 1–2 minutes until the sauce coats the back of a spoon. This béchamel is the structural backbone of the soufflé: it adds body so the final dish is tender yet stable.

Remove the béchamel from the heat and let it cool for 1–2 minutes. Whisk in the egg yolks one at a time (tempering them into the warm sauce prevents scrambling) until fully incorporated. Stir in the grated Gruyère, salt, pepper, and a tiny pinch of nutmeg. The cheese melts into the warm béchamel, adding savory depth and the classic nutty aroma.

In the large, clean bowl for the whites, add the cream of tartar (if using) and beat the egg whites to soft peaks using a hand or stand mixer. Start on medium speed until the foam forms, then increase speed. Soft peaks should fold over gently when you lift the whisk — properly aerated whites are what give the soufflé its lift and light texture.

Temper the whites: stir a generous spoonful of the whipped whites into the cheese béchamel to lighten it — this makes folding easier and keeps the mixture from deflating. Then gently fold the remaining whites into the béchamel in three additions using a rubber spatula: cut down through the center, scoop up and over, and rotate the bowl. Fold just until streaks disappear; over-folding knocks out air and yields a denser soufflé.

Spoon the mixture into the prepared ramekins, filling to about 3/4–7/8 full (leave room for rise). Smooth the tops with a wet offset spatula and run your thumb around the inside rim of each ramekin to create a small groove — this helps the soufflé to rise straight rather than dome over unevenly.

Place the baking sheet with ramekins into the preheated oven and immediately reduce the temperature to 375°F (190°C). Bake for 20–25 minutes until puffed, deeply golden on top, and the centers still wobble slightly when nudged. Do not open the oven during baking — a sudden rush of cold air will make them collapse. The temperature reduction after placement gives a gentle, even rise.

Serve immediately: soufflés begin to deflate within minutes of coming out of the oven. Serve directly in their ramekins, optionally with a simple green salad dressed with lemon vinaigrette to cut the richness. The first bite should be light and airy with molten cheesy pockets from the Gruyère.