In a large bowl, cream butter and sugar until light and fluffy, 5-7 minutes. Beat in egg, orange zest and vanilla. In another bowl, whisk flour and cornstarch; gradually beat into creamed mixture., Divide dough in half. Roughly shape each portion into a 7-in. roll along the long end of a 14x8-in. sheet of waxed paper. Tightly roll waxed paper over dough, using the waxed paper to mold the dough into a smooth roll. Place waxed paper-covered roll in an airtight container; freeze 30 minutes or until firm, or refrigerate overnight. , Preheat oven to 350°. Sprinkle pistachios on a rimmed baking sheet. Unwrap and roll each roll of dough in pistachios. Cut dough crosswise into 1/4-in. slices. Place 1/2 in. apart on parchment-lined baking sheets. Bake 6-8 minutes or until bottoms are light brown. Cool slightly on pan. Remove from pans to wire racks to cool completely., In a small bowl, combine confectioners' sugar, orange juice and butter until smooth. Spread over cookies. If desired, sprinkle with additional pistachios. Let stand until set.