* Place in a bowl flour, butter, sugar, hard boiled yolks, and lemon rind.
* Combine the ingredients until forming a smooth dough.
* Wrap the dough in plastic and place in the refrigerator for about 30 minutes.
* Preheat oven 375 F (190 C). On a lightly floured work surface, flatten the dough with a rolling pin until about 1/4 inch (5 mm) thick. Avoid heating the dough, otherwise will become inconsistent and difficult to work.
* Cut the cookies with a round cookie cutter.
* With the help of a spatula (the dough is very soft and fragile) transfer the cookies to a cookie sheet lined with parchment paper.
* Bake for about 12 - 13 minutes, until fully cooked but still pale.
* Remove the cookies from the pans immediately and transfer to a rack to cool.
***NOTE***
Watch the cookies closely. Don’t color them: They burn very easily.