Melt butter, cook flour 2 minutes, whisk in warm milk in 3 batches. Add salt and pepper, turn off heat, whisk in yolks one at a time, then melt through gruyere over low heat. Beat egg whites and cream of tartar to firm peaks, starting on medium for 2 minutes, then increasing to high. Whisk 1/4 of the whites into Mornay Sauce, then fold through remainder in 3 batches. Fill butter brushed ramekins to 1cm/0.4” from rim, clean inside of rim. Bake at 200°C/400°F (180°C fan) for 20 minutes. No peeking! Serve immediately.

Preheat oven to 200°C / 400°F (180°C fan-forced). A properly hot oven is crucial, it's what gives the soufflé its famous lift. A cold oven = a sad soufflé.

Prepare the ramekins (Note 4) – Brush the insides of 4 (325ml / 1 1/3 cups) ramekins generously with melted butter (use all the butter). No need to brush upwards (see post for why). Set on a baking tray.

Heat the milk - Place the milk in a small saucepan over medium heat. Warm it until it’s steaming but not boiling. Set aside.

Make the roux - While the milk is heating, let’s start our roux. In a large saucepan, melt the butter over medium heat. Once melted, add the flour. Whisk it gently, it will be thick and pasty at first like a wet sand. Cook, whisking constantly, for about 2 minutes. It will thin up and become smooth and glossy. Make sure to use the whisk to get into the corners of the pot and don’t let it brown. Reduce heat if you think it’s too high.

Make Béchamel - Add a third of the hot milk to the roux while whisking vigorously, it will thicken very quickly. Add half of the leftover milk, whisk until smooth and then pour the rest of it along with the salt and pepper. Keep whisking for 30 seconds to a minute until thickened and smooth, lumps will go away. It will be thick enough so you can draw a path with a spatula in the middle, it should hold its shape for a moment before slowly sinking back.

Turn it into a Mornay sauce - Turn off the heat and whisk in the egg yolks, one at a time, until fully combined (don't worry the egg yolks won't cook). Then, turn the heat back on to medium and whisk in the grated Gruyère until melted through. Scrape down the sides and cover with a lid. Set aside.

Beat the egg whites, gently at first - Add egg whites and the cream of tartar in a large bowl. Using a hand mixer or a stand mixer fitted with a whisk attachment, beat the egg whites on medium speed for 2 minutes then high speed for 30 seconds until you get firm peaks. Starting slowly helps the protein structure to develop properly. Rush it and you risk unstable, foamy whites that collapse when folded in.

Fold gently - Give the Mornay a quick whisk to make it smooth again then add in a quarter of the egg whites and whisk it in to loosen it. Then gently fold a third of the remaining egg whites in using a rubber spatula until mostly incorporated. Use slow, sweeping motions from the bottom of the pot, you want to keep as much air in as possible. (See video at 2.15 minutes) Rough folding = flat soufflé. Few lumps are ok. Repeat twice more with the rest of the egg whites.

Fill the ramekins - Pour the mixture evenly into the ramekins using a ladle or a large serving spoon, filling to about 1cm / 0.4" below the rim. Smooth the tops with the back of a teaspoon dipped in water, no need to be 100% meticulous here, just get rid of big bumps. Run your thumb around the inside rim of each ramekin to clean it, this gives the soufflé a clean edge to rise against.

Bake - Place the ramekins on a baking tray and bake for 20 minutes until tall and golden. Do not open the oven door while it's baking, the sudden drop in heat will make them sink. When it's cooked, the soufflés should jiggle a bit when you shake the ramekin.

Serve immediately - As soon as they’re out, serve straight away. Soufflés don't wait. So make sure all your family/guest are seated so they can enjoy the show!