Beat the egg whites with the electric whisker at high speed to very stiff peaks;
Put the butter in the microwave for a few seconds: it must be soft but not liquid (make the“finger test”: the butter should keep its shape but soft to the touch);
In a big mixing bowl combine the softened butter and the powdered sugar with a stand mixer until a fluffy smooth batter; watch out for clumps;
In another bowl, mix the flour with salt and vanilla powder; set aside;
Combine the egg whites you have beaten until stiff peaks to the butter and sugar mix a little at a time with upwards motions not to deflate the batter. Usa a spatula to add the whites a little a time with upward motions not to deflate the batter; combine well and then add another spoon until everything is evenly mixed;
Gently pour the flour a little at a time- here again, combine well and then add another spoon until everything is evenly mixed;
Refrigerate for about 30 minutes, the batter should get more compact;
In the meanwhile pre-heat the static oven at 170°C (338°F) or the fan one at 150° (302°F); if you like golden brown cookies increase the temperature to 1780°C (356°F) but be careful as lingue di gatto may get burnt very easily;
Pour the batter into a piping bag with a 1/2 inch smooth steel tip (you may use plastic ones was well),
Line your biggest baking tray with parchment paper, and pipe out 8cm (3 inches) stripes approximately 2,5 cm apart (1 inch) because they will grow in the oven, and they must not touch;
Be careful when using the piping bag: try to make regular stripes with the same length and thickness; your cookies will bake evenly;
For softer cookies pipe out more batter and make thicker stripes; if you opt for super crumbly lingue di gatto, place a thinner layer of batter, but keep the cooking time under strict control;
Place the baking tray onto the middle rack of the oven;
Bake for about 8-10 minutes until the edges of the cookies get light golden (they must not be brown or burnt, so don ’t leave the kitchen for any reasons!);
During the baking they will rise (because they get dry) and then they will flatten;
When the baking time is over, I usually increase the temperature to 180°C (356°F) for 1 minute in the fan oven to make them crumbly and dry;
If they look too pale, open the oven door and touch them with a finger (no, they don’t burn)- if their texture is elastic and compact, they are almost done;
Take lingue di gatto cookies out of the oven and with a spatula delicately transfer them onto a cooling rack;
Give way to the temptation: taste one hot cookie, …just imagine how scrumptious it is!