||Preheat oven to 325 degrees. Butter/spray a 12″x8″ rimmed baking sheet. Line with parchment paper, letting it hang over the long sides.Whisk together flour, cocoa, cinnamon, salt, and baking soda. Set aside.
Beat together butter and sugar on medium speed until light and fluffy. If you don’t have superfine sugar, process granulated sugar in a food processor for 15 seconds. Add flour mixture and mix until just combined.
Spread dough into baking sheet. Chill dough in refrigerator or freezer for about 15 minutes or until firm. Prick 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 top of shortbread with granulated sugar, covering completely. Cool completely in the pan.