1. Whisk together flour and salt; transfer flour mixture to food processor with chopped chocolate and process for 1 minute or until chocolate is finely ground; set aside.
2. Stir together espresso powder and 2 teaspoons hot water until espresso dissolves; set aside. Beat butter for 3 minutes. Add granulated sugar and beat 2 minutes or until well blended. Beat in espresso mixture. Reduce speed to low and gradually beat in flour mixture.
3. Press dough into a 9-inch round fluted nonstick tart pan Score into 16 pie-shaped wedges without cutting completely through dough. Cover with plastic wrap and chill at least 2 hours.
4. Heat oven to 350 degrees F. Uncover shortbread and bake at 350 degrees F. for 30 minutes. Remove to a wire rack and remove side of pan. Dust with confectioners' sugar, if desired. Cut through score marks before serving.