In a bowl, whisk together flour, cocoa powder, salt, and ground cardamom; set aside. In a mixing bowl, beat together butter and sugar with orange zest until fluffy. Beat in egg and vanilla, then stir in the flour mixture. Blend until just combined, scraping sides of bowl as necessary.
Transfer dough to a square of parchment, form into a log 2 inches in diameter and roll tightly in parchment. Refrigerate until solid, about 1 hr, rotating every 20 min to maintain shape.
Heat oven to 350 degrees F. Lightly brush the dough log with egg white. Place sanding sugar and star sprinkles in a rimmed pan and roll log in sprinkles until coated well. Or you can apply luster dust to the log with a soft brush. (We divided the log in thirds, and decorated each with different sprinkles.) Slice the log into rounds that are about a 1/4 inch thick. Place on parchment-lined baking sheets an inch apart.
Bake until cookies are set and there is slight resistance when you touch centers, 9 to 10 minute Cool on pans. Store airtight up to 1 week. Makes 40 cookies.