source: Martha Stewarts Baking Handbook
3 sticks (1 1/2 cups) unsalted butter, plus more for pan
2 1/2 cups plus 2 tablespoons all-purpose flour
4 1/2 tablespoon Ducth-process cocoa powder
heaping 1/2 teaspoon ground cinnamon
1/2 teaspoon salt
1/4 teaspoon baking soda
1 cup superfine sugar
Granulated sugar for springkle
Preheat the oven to 325 F. Butter a 12 by 8 inch rimmed baking sheet, and line with parchment paper, leaving a 1 inch overhang on long sides, set a side.
In medium bowl, whisk together flour, cocoa, cinnamon, salt and baking soda until combined.
In an electric mixer, beat the butter and superfine sugar on medium-high speed until light and fluffy, 3-4 minutes, scraping down the sides of bowl.
Add flour mixture, and beat on medium speed until combined
Spread dough in prepared baking sheet, chill in freezer of refrigerator until dough is firm, about 15 minutes.
Prick dough all over with fork, bake until just firm about 20 minutes.
transfer to a wire rack, while still hot, use a knife to cut the shortbread
sprinkle with granulated sugar, and let cool completely.