Beat quickly egg whites with lemon juice, almond extract and vanilla seeds.They have to be just foamy and bubbly, not whipped. Add almond meal and icing sugar, mix with a spoon and wrap this dough in cling film.Let it rest in fridge for a minimum of 24 hours, two days is even better (but no more than three days).
After this time roll the dough over a surface sprinkled with icing sugar, form a long sausage and cut pieces from it.
Drop each piece into icing sugar then give it the classic diamond shape and put it on a baking tray.
Before baking pass slightly wet finger over the surface of each cookie and sprinkle with icing sugar.
Bake at 170°C/ 360°F until cracks appear, then low to 150°C/ 300°F for 5-6 minutes more.
Let them cool completely before touching, they will last up to two weeks in a tin box.