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Preheat oven to 325 degrees F.
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Butter/spray a 12″x8″ rimmed baking sheet. Line with parchment paper, letting it hang over the long sides.
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Whisk together flour, cocoa, cinnamon, salt, and baking soda. Set aside.
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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.
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Add flour mixture and mix until just combined.
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Spread dough into baking sheet. Chill dough in refrigerator or freezer for about 15 minutes or until firm.
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Prick dough all over with a fork. Bake for about 20 minutes or until firm.
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While still hot, cut into 4″x1″ pieces.
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Sprinkle top of shortbread with granulated sugar.
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Cool completely in the pan.