Preheat oven to 325°F (160°C). Line a 12”x 8” rimmed baking sheet or quarter sheet pan with parchment paper, letting it hang over two opposite sides.
Whisk together the flour, cocoa, cinnamon, salt, and baking soda. Set aside.
Using an electric mixer on medium speed, beat the butter and sugar until light and fluffy. Add the flour mixture, and mix until just combined.
Spread the dough into the prepared pan. Chill the dough in the refrigerator or freezer for about 15 minutes or until firm.
Prick the dough all over with a fork. Bake for about 20 minutes or until firm.
While still hot, cut into 4”x1” pieces. (A pizza cutter works wonderfully for this.)
Sprinkle the top of the shortbread with granulated sugar, covering completely. Cool completely in the pan.
Notes
*If you don’t have superfine sugar, process granulated sugar in a food processor for 15 seconds. Read more here: How to Make Superfine Sugar
Recipe adapted from Martha Stewart's Baking Handbook.
To store: Chocolate shortbread will last about 4 days when stored in an airtight container at room temperature. You can also store them in an airtight container in the fridge to extend their shelf life to a week.
To freeze: Place the shortbread cookies in a freezer bag or airtight container, separating layers with parchment paper if necessary. Freeze for up to 3 months; when you’re ready to enjoy them, let the shortbread thaw on the countertop at room temperature.