1. Preheat oven to 325F (160C)
2. Using an electric mixer, beat together butter and sugar until light and fluffy. Beat in vanilla. In separate bowl, combine flour, baking soda, salt, orange zest and chocolate; using wooden spoon, gradually stir into butter mixture until well combined.
3. Divide dough in thirds, pat each into 6" (15 cm) circle onto parchment paper lined cookie sheets. Using paring knife, without cutting all the way through dough, score each circle into 8 wedges.
4. Bake in centre of oven for 15 to 20 minutes, or until lightly golden. Cool on pan on racks for 5 minutes. Cut through score marks. Drizzle cooled cookies with melted chocolate.