Beat shortening and butter at medium speed of an electric mixer until creamy; gradually add sugar, beating until light and fluffy.
Scrape vanilla bean seeds into mixture; discard pod. Beat in egg yolk. Add flour and salt, beating at low speed just until blended. Stir in grated chocolate. Press dough into an ungreased 15" x 10" x 1" jellyroll pan.
Bake at 325° for 25 minutes. Let cool slightly in pan. Cut shortbread into 3" x 1 1/4" strips, using a fluted pastry wheel. Let cool completely in pan.
Shortbread Wedges: Press half of dough into an ungreased 9" round cakepan. Bake at 325° for 28 to 30 minutes. Let cool completely in pan. Cut shortbread into thin wedges. Repeat procedure with remaining dough. Yield: 2 dozen