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1
Sift the flour, cocoa powder, cinnamon and salt together and keep close at hand. Place the butter in a mixer fitted with the paddle attachment and beat on medium speed to soften. Gradually add the sugar and vanilla and continue to beat, scraping down the sides of the bowl as needed, until the mixture is smooth and creamy, but not airy. Reduce the mixer speed to low and add the flour mixture, mixing only until the ingredients are just mixed-no more. Alternatively, you can remove the bowl from the mixer and stir the flour into the dough with a rubber spatula. The point is to mix the dough as gently and as little as possible-this light touch is what will give the cookies their characteristic crumbly texture. As soon as the last of the flour is no longer visible, divide the dough in half, shape each half into a ball, wrap the balls in plastic wrap and chill for 30 minutes.
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2
Working on a smooth surface, form each piece of dough into a log that's about 1 1/2 inches thick and 7 1/2 inches long. (Aim to get the thickness right and the length will be fine.) To get a solid log, one without the commonly found hole in the center, use the heel of your hand to gently flatten the dough, then flatten the dough lightly each time you fold it over on itself to make the log. Assured that the log is solid, you can roll it gently under your palms to smooth it out. Wrap the logs in plastic and chill for 1 to 2 hours. (The dough can be made to this point, wrapped airtight, and stored in the freezer for up to 2 months.)
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3
Position the racks to divide the oven into thirds and preheat the oven to 350u00b0F. Line two baking sheets with parchment paper and set them aside. In a small bowl, whisk the egg yolk until it is smooth and liquid enough to use as a glaze; keep it close at hand. Spread some of the sugar out on a piece of wax paper.
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4
Remove the logs of dough from the refrigerator, unwrap them and brush them very lightly with a small amount of the egg yolk. Roll the logs in the sugar, pressing the sugar gently to get it to stick, if necessary, then, using a sharp slender knife, slice each log into cookies 1/2-inch thick. Arrange the cookies on the baking sheets, leaving about an inch of space between each cookie, and bake for 15 to 18 minutes, rotating the pans front to back and top to bottom at the midway mark, until the cookies are just firm to the touch. Transfer the cookies to racks to cool to room temperature.
