Preheat the oven at 350°F (180°C) and cover a baking sheet with parchment paper.
In a bowl, work the butter and sugar together with a spoon. Add in the egg whites, then the vanilla, and finally the flour. Mix well until forming a homogeneous batter.
Using a piping bag or spoon, form narrow rectangles of batter on the lined baking sheet, taking care to space them well.
Bake for about 6 min, until the edges start to turn gold. The cookies will be soft when you take them out of the oven, wait for them to harden as they cool to move them.
Eat within the day, as they are only crunchy when fresh.