Butter a 12-by-8-inch rimmed baking sheet, and line with parchment paper, leaving a 1-inch overhang on long sides; set aside.
In a medium bowl, whisk together flour, cocoa, cinnamon, salt, and baking soda until combined.
In an electric mixer fitted with the paddle attachment, beat the butter and superfine sugar on medium-high speed until light and fluffy, 3 to 4 minutes, scraping down the sides of the bowl.
Add flour mixture, and beat on medium speed until just combined.
Using a small offset spatula, evenly spread dough in prepared baking sheet.
Chill in the freezer or refrigerator until dough is firm, about 15 minutes.
Prick dough all over with a fork; bake until just firm to the touch, about 20 minutes.
Transfer to a wire rack.
While still hot, use a large knife to cut shortbread into 4-by-1-inch pieces.
Sprinkle with granulated sugar.
Cool completely in the pan.
Shortbread can be kept in an airtight container at room temperature for up to 1 week.