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 two 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.
Store in an airtight container up to 1 week.