Grind the powdered sugar and butter well, then add the yolk and knead the dough using a mixer. Act quickly.
Sift the flour into the mixture and bring the mixture until smooth.
Add nuts and/or dried fruites if use them.
Roll the resulting dough into balls the size of a walnut. Place the balls on a baking sheet lined with baking parchment.
Bake the cookies for 15 minutes, but starting from the 10th minute, start to control the process: you need to get a barely golden brown bottom, otherwise the cookies will be too dry.
Dip the still hot cookies into powdered sugar. This will give it a nice sweet crust.