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1
Place the chocolate in a small saucepan over low heat, stirring occasionally until its melted and smooth; set the pan aside.
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2
(Alternatively, melt the chocolate in the microwave, stirring at 30-second intervals to avoid burning.)
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3
Whisk the measured flour, baking powder, and salt together in a large bowl to aerate and combine; set aside.
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4
Stir the milk and vanilla together in a small bowl; set aside.
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5
Place the butter and sugar in the bowl of a stand mixer fitted with a paddle attachment and beat on medium speed until lightened in color and fluffy, about 3 minutes.
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6
Stop the mixer and scrape down the beater and the sides of the bowl with a rubber spatula.
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7
Add the egg yolks and beat until combined, about 30 seconds.
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8
Stop the mixer and scrape down the beater and the sides of the bowl with the rubber spatula.
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9
Add a third of the flour mixture and turn the mixer to low speed, mixing until the flour is just incorporated.
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10
Add half of the milk mixture and mix until just incorporated.
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11
Continue with the remaining flour mixture and milk mixture, alternating between each and ending with the flour, until all of the ingredients are incorporated and a soft dough has formed.
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12
Remove half of the dough (about 1 pound 2 ounces) to a lightly floured work surface.
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13
Remove 2 tablespoons of the remaining dough in the stand mixer and add it to the dough on the work surface (this is to make the volume of both the chocolate and vanilla doughs even).
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14
Pour the melted chocolate into the stand mixer and beat on low speed until completely incorporated and no streaks of chocolate remain, about 30 seconds; set aside in the mixer bowl.
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15
Divide the vanilla dough into 2 equal portions.
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16
Lightly flouring the work surface and your hands as needed, shape each portion into a log that is 1 1/2 inches square and about 6 inches in length, using a bench scraper or flat spatula to form sharp corners and flat sides; set aside.
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17
Remove the chocolate dough from the mixer bowl to the work surface and divide it into 2 equal portions.
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18
Lightly flouring the work surface and your hands as needed, shape each portion into a log that is 1 1/2 inches square and about 6 inches in length, using the bench scraper or flat spatula to form sharp corners and flat sides.
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19
Wrap each log tightly in plastic wrap and refrigerate until just firm but still pliable, about 1 hour.
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20
Remove all the dough logs from the refrigerator.
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21
Unwrap 1 log of vanilla dough and 1 log of chocolate dough and place them on a cutting board.
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22
Cut the vanilla dough in half lengthwise.
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23
Turn each half so that it is cut-side down, then cut each piece in half lengthwise again (you should now have 4 [3/4-inch-square, 6-inch-long] pieces of dough); set aside.
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24
Repeat with the chocolate dough.
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25
Lay a strip of chocolate dough next to a strip of vanilla dough lengthwise so that they are touching.
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26
Top the chocolate strip with a vanilla strip, and top the vanilla strip with a chocolate strip.
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27
Gently press all 4 strips together so that they adhere to one another, using the bench scraper or flat spatula to form sharp corners and flat sides.
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28
Rotate the dough and gently press each side into the cutting board to finish flattening and creating sharp corners.
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29
You should now have a 1-1/2-inch-square checkerboard log of dough.
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30
Repeat with the remaining 4 strips of dough.
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31
Repeat cutting and assembling the remaining 2 logs of vanilla and chocolate dough until you have 4 checkerboard logs total.
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32
(Reserve the cutting board and knife.)
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33
Wrap each log tightly in plastic wrap and refrigerate until firm, at least 1 hour.
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34
Heat the oven to 350 degrees F and arrange the racks to divide the oven into thirds.
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35
Line 2 baking sheets with parchment paper and set aside.
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36
Remove 2 checkerboard logs from the refrigerator, unwrap, and place them on the cutting board.
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37
Slice each log crosswise into 1/4-inch-thick cookies and place them on the prepared baking sheets (about 25 per sheet), spacing the cookies about 1 inch apart.
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38
Bake both sheets for 7 minutes.
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39
Rotate the pans from top to bottom and front to back and continue baking until the cookies are set around the edges, about 5 to 6 minutes more.
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40
Place the baking sheets on wire racks and let the cookies cool on the sheets for 3 minutes.
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41
Using a flat spatula, transfer the cookies to the wire racks to cool completely.
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42
Let the baking sheets cool to room temperature (reserve the parchment paper) and repeat with the 2 remaining logs of dough.
