Natural cocoa powder will work in this recipe, but we found that Dutch-processed yields the best chocolate flavor. Espresso powder provides complexity, but instant coffee can be substituted in a pinch. The dough can be wrapped tightly in plastic wrap and stored in the refrigerator for up to three days or in the freezer for up to two weeks. Defrost frozen dough in the refrigerator overnight, then let stand at room temperature until firm yet malleable, about 30 minutes. The cookies are refined enough to serve plain, but a dusting of sifted confectioners’ sugar or chocolate glaze is a nice touch. Baked cookies can be stored in an airtight container at room temperature for up to three days but should be dusted with sugar or glazed the day they are served.