Combine granulated sugar and orange zest in a medium bowl. Using your fingers, rub zest into sugar until zest is evenly dispersed and sugar is uniform in color and feels like wet sand, 1 to 2 minutes. Set aside. Make the cookie dough:

reheat oven to 350°F with racks in upper third and lower third positions. Line 2 baking sheets with parchment paper; set aside. Beat butter, egg, salt, baking soda, and 1 1/2 cups orange sugar with a stand mixer fitted with the paddle attachment on medium-high speed until mixture is light and fluffy, 2 to 3 minutes. Add flour and cocoa; beat on low speed until just incorporated, about 30 seconds. Using a 2-inch (about 1 1/4-ounce) cookie scoop, scoop dough evenly into balls (about 2 1/2 tablespoons each). Roll each portion into a ball between your palms.

Place remaining 1/2 cup orange sugar in a small bowl; roll dough balls in orange sugar until well coated. Arrange dough balls about 2 1/2 inches apart on prepared baking sheets. Using the palm of your hand, gently press each dough ball to flatten to about 3/4 inch thickness (about 2 inches in diameter).

Bake cookies in preheated oven until tops are crackled and edges are softly set, about 12 minutes, rotating baking sheets between top and bottom racks halfway through baking time. Remove from oven. Let cookies cool on baking sheets 5 minutes. Transfer cookies to a wire rack, and let cool completely, about 30 minutes. Make the Campari icing:

Whisk together powdered sugar and Campari in a small bowl until mixture is smooth and glaze is thick but pourable.

Drizzle cooled cookies evenly with Campari icing. If desired, garnish cookies with sparkling sugar. Let cookies stand until icing has hardened, about 30 minutes.