1Preheat oven to 425°F. Separate eggs carefully; if there is the tiniest bit of yolk in the whites they will not beat properly, so be sure to separate them thoroughly and to put the whites into an extremely clean, dry bowl. You will need all of the whites but only 4 yolks. Eggs are easiest to separate when cold, but they are easier to beat at room temperature so do this step first to allow the whites to warm up.
2Butter a 1 1/2 quart soufflé mold very well. Throw in a handful of sugar and shake the soufflé dish until it has a thin coating of sugar. Shake out excess. Set aside.
3Melt the butter in a large, heavy bottomed pan. Add the flour and whisk until well blended. Slowly stir in milk. Cook, stirring, until the mixture has almost reached the boiling point and has become thick and smooth. Add lemon juice and sugar and cook for 2 minutes more. Remove from heat, add vanilla, and cool slightly. Add 4 egg yolks, one at a time, beating to incorporate each one before adding the next. Add lemon zest, then return the pan to the stove and cook, stirring constantly, for 1 minute more over medium heat. Remove and let cool.
4Add a pinch of salt to the 6 egg whites and beat with a clean beater until they form soft peaks. Stir a quarter of the egg whites into the sauce, then carefully fold in the rest.
5Pour into the soufflé mold and set on the middle rack of the oven. Turn heat down to 400 degrees F and bake for 25 to 30 minutes or until the top is nicely browned and the soufflé has risen about 2 inches over the top of the dish.