Wrap these cookies in metallic foils to make cookies inspired by Hanukkah gelt.
In the bowl of an electric mixer fitted with the paddle attachment, cream butter and brown sugar until light and fluffy, about 3 minutes. Add egg; mix until well combined.
In a large bowl, whisk together cocoa powder, flour, salt, and orange zest, if using. Add flour mixture to egg mixture; continue mixing until the ingredients are just combined, scraping down sides of the bowl.
Scrape dough onto a piece of plastic wrap; flatten into a disk. Wrap, and chill at least 1 hour or overnight.
Heat oven to 350 degrees. Line 2 baking sheets with parchment paper. Place granulated sugar in a small bowl.
On a lightly floured work surface, roll out dough to a 1/4-inch thickness. Using a 2-inch-round cutter, cut out cookies. Carefully press each cookie into granulated sugar. Transfer cookies to sheets, spaced 1/2 inch apart.
Bake until cookies are firm, about 12 minutes. Cool on a wire rack. Wrap in foil.