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1
Sift flour, cocoa powder, cinnamon, and salt together.
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2
Put butter in a stand mixer bowl w/ mixer fitted with paddle attachment; beat on medium speed to soften it.
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3
Gradually add the sugar and vanilla and continue to beat, scraping down the bowl as needed, until the mixture is smooth and creamy but not airy.
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4
Decrease mixer speed to low and add in the flour mixture, blending only until the ingredients are just combinedno more.
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5
Alternativeremove 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.
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6
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; chill for 30 minutes.
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7
Working on a smooth surface, form each piece of dough into a log that is about 1 1/2 inches thick and 7 1/2 inches long.
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8
To get a solid log, one without that 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.
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9
Assured that the log is solid, you can roll it gently under your palms to smooth it out.
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10
Wrap logs in plastic; chill for 1-2 hours.
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11
Position oven racks to divide the oven into thirds; preheat oven to 350.
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12
Line two baking sheets with parchment paper; set aside.
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13
In a bowl, whisk the egg yolk until it is smooth and liquid enough to use as a glaze; spread some sugar out on a piece of waxed paper.
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14
Remove the logs of dough from the refrigerator; unwrap them; brush them very lightly with a small amount of egg yolk.
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15
Roll logs in the sugar, pressing gently on the sugar to get it to stick if necessary.
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16
Using a sharp slender knife, slice each log into cookies 1/2 inch thick.
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17
Arrange cookies on the baking sheets, leaving about an inch of space between each one.
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18
Bake 15-18 minutes, rotating the pans front to back and top to bottom at the midway point, until the cookies are just firm to the touch.
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19
Transfer cookies to racks to cool to room temperature.
