Cream butter, sugar, orange peel, and salt in the mixer at high speed for 1 to 2 minutes, untill light and fluffy. Add the egg and beat 1 minute more at medium speed. Add the chocolate and the flour and beat at low speed just enough to combine. Divide the dough, flatten each half into a 15 cm circle, wrap in plastic wrap and chill for at least 2 hours or, better yet, overnight. When ready to proceed, preheat oven to 190ºC . Roll out each circle of dough, one at a time, between sheets of lightly flowered waxed paper until scant 1 cm thick, sprinkle with flour once during rolling. Carefully pel off the top piece of paper and, using 6 cm round cutters, cut the dough-still on the bottom sheet of wax paperinto rounds but do not lift from paper. Slide the paper of dough on a cookiesheet and set in the freezer for 5 minutes. Than use small spatula to remove the cookies and space 3 cm apart on a lightly greased baking sheet. Bake the cookies in the middle of the oven for 8-10 minutes, or until lightly browned on the edges. Transfer at once to wire racks. For the glaze: whisk the powdered sugar with the juices until smooth. Using pasrty brush, brush the glaze on the still warm cookies. As soon as glaze is hard you can store them with waxpaper between the layers in airtight containers.