In a bowl stir together flour and salt. In another bowl with an electric mixer at medium speed beat butter until smooth. Gradually add confectioners` sugar to butter, beating on low speed (to prevent sugar from flying out of bowl), scraping down side of bowl with a rubber spatula occasionally. Beat in tangerine or orange zest and vanilla. Gradually beat in flour mixture until combined well. Divide dough into thirds. Roll out each third between sheets of wax paper into a 10-inch round (about 1/8 inch thick) and stack rounds in wax paper, on a baking sheet. Chill rounds until firm, at least 2 hours, and up to 3 days. Preheat oven to 300°F and line 2 large baking sheets with parchment paper. In a bowl toss together topping ingredients. Transfer 1 round, still between sheets of wax paper, to a work surface. Remove top sheet of wax paper from round (keep remaining 2 rounds chilled) and replace it loosely. Flip over wax-paper-enclosed dough round and remove and discard sheet of wax paper now on top (this process prevents cookies from sticking to wax paper). Working quickly, with a 2-inch fluted round cutter cut out rounds, arranging about 1/2 inch apart on baking sheets. (If dough becomes too soft to work with, freeze or chill on wax paper until firm.) Reroll scraps and make more cookies with remaining rounds in same manner. Spoon 1/2 to 1 teaspoon topping onto center of each cookie. Bake cookies in batches in lower third of oven until edges are golden but centers still pale, 15 to 18 minutes. Cool cookies on baking sheets on racks 5 minutes and transfer with a spatula to racks to cool completely. Cookies keep, layered between sheets of wax paper in airtight containers at room temperature, 1 week.