Cheese soufflé

1. Preheat the oven to 180°C (350°F)
2. In a saucepan, melt 30 ml (2 tbsp) of butter over medium heat.
3. Add 50 ml (3 1/2 tbsp) of flour.
4. Cook while stirring constantly with a wooden spoon for 2 to 3 minutes or until the roux whitens.
5. With a whisk, add the cold milk and cook while stirring frequently. Season.
6. Cook until the sauce boils and thickens. Note: This béchamel must be thick, and it is important that it remain that way for the  soufflé to be a success.
7. Remove from the heat.
8. Off the heat, quickly add 3 egg yolks*, one at a time, stirring well (be sure to reserve the whites).
9. Add the Gruyère and stir well. Place in a bowl. Set aside.
10. Line 2 soufflé moulds, 12 cm (4 3/4 in.) wide by 10 cm (4 in.) high, with the rest of the butter and flour. Note: Once this operation is done, it is important not to touch the inside of the moulds as it would compromise the success of your recipe.
11. In a large bowl, place 4 egg whites. Add a pinch of salt.
12. With a whisk, whip the whites until stiff and firm.
13. In another bowl, using a spatula gently fold 1/3 of the whites into the béchamel.
14. Fold the rest of the egg whites into this mixture, and stir gently.
15. Place the preparation in the moulds, filling up to 3/4 of each mould.
16. Cook in the oven for about 25 minutes.
17. Serve immediately.

Note:

 It is essential not to open the oven door before the end of the cooking time: any temperature decrease may spoil all your work!

The fourth egg yolk may be reserved for another recipe.