Position a rack in the center of the oven and preheat to 350ºF.
In a medium bowl, stir together the flour and salt. In a large bowl, using a hand-held electric mixer set at medium speed, beat the butter for about 30 seconds, until creamy. Beat in the sugar until well combined. Beat in egg, orange zest and vanilla until well combined. On low speed, gradually beat in the flour mixture just until combined.
Scrape the dough into a pastry bag fitted with a plain pastry tip (such as Ateco Nr. 4 of Nr. 5). Holding the tip of the pastry tip close to the baking sheet, slowly pipe 2 x 1/2-inch strips onto the baking sheet, leaving 1 inch between cookies. Bake the cookies for 11 to 13 minutes, or until the undersides are lightly browned. Cool the cookies on the baking sheet set on a wire rack for 1 minute. Using a metal spatula, transfer the cookies to a wire rack and cool completely.
Place the cooled cookies on a wax paper-lined baking sheet. Fill two small paper cones or pastry bags fitted with 1/8-inch plain pastry tip (such as Ateco Nr. 1) each with the melted chocolates. Pipe random lines of semisweet chocolate over the cookies. Refrigerate 5 minutes or until chocolate is firm. Pipe random lines of white chocolate over the semisweet chocolate. Refrigerate 5 minutes or until chocolate is firm. Repeat the piping of the semisweet chocolate and white chocolate. Refrigerate 5 minutes or until chocolate is firm. Store the cookies in an airtight container at cool room temperature for up to 5 days. YIELD: approx. 4 1/2 dozen cookies