Mix the egg whites with the salt powder. Gradually add the sugar and continue mixing. Then add the butter at room temperature, the raw yolk, the boiled yolks passed through a sieve, a cup of flour mixed with baking powder and vanilla sugar.
Homogenize. Then add the rest of the flour, gradually, kneading by hand, until you get a firm dough but with which you can still work in a biscuit machine (ie not to be extremely hard).
When you're done you can put the dough in a good posh, (as I did and I worked like a shovel =)) that's how my hand hurt :))) or you can put it in the meat grinder to which you can attach the shape for cookies or you can shape balls by hand (how easy it would have been to do so :)))))
about 40 pcs come out. Regardless of the method you choose, put the cookies in the tray in which you put baking paper greased with a little butter.
Bake in the preheated oven, at 170-175 degrees, for 13-15 minutes. You don't have to brown them, just let the sole be slightly brown. You can break one to check.
You can powder them with sugar or not, you can glaze chocolate or any other glaze or you can stick them in pairs with cream.