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1
Sift both flours, baking powder, and salt into a large bowl; set aside.
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2
In the bowl of an electric mixer fitted with the paddle attachment, beat the egg yolks and sugar on medium-high speed until doubled in volume and pale yellow, 2 to 3 minutes, scraping down the sides of the bowl as needed.
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3
Add the butter in four parts, beating until completely combined after each, 1 to 2 minutes total.
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4
With the mixer on low speed, add flour mixture, beating until combined.
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5
Turn out the dough onto a lightly floured surface.
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6
Divide in half, and flatten into disks; wrap each in plastic.
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7
Refrigerate at least 30 minutes or up to 1 day.
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8
Preheat the oven to 325F, with racks in the upper and lower thirds.
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9
Line two large baking sheets with parchment paper; set aside.
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10
Generously dust a large piece of parchment paper with flour.
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11
Place one disk of dough in the center, and roll out to slightly thicker than 1/4 inch.
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12
To prevent sticking while rolling, occasionally run a large offset spatula under dough, and add more flour to the top or bottom.
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13
Transfer parchment and dough to a baking sheet; chill until firm, about 15 minutes.
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14
Repeat with remaining disk.
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15
(You can stack sheets of dough in the freezer.)
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16
Remove one sheet of dough and parchment from the freezer, and transfer to a clean work surface.
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17
Using a 2 1/2-inch round cookie cutter, cut out rounds and place about 1 1/2 inches apart on prepared baking sheets.
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18
Gather the scraps; roll out again and cut out more rounds.
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19
In a small bowl, whisk together the whole egg and 1 tablespoon water; brush over tops of rounds.
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20
Let stand 5 minutes and brush again.
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21
Using a paring knife, score each round in a shallow lattice pattern, making sure not to cut all the way through the dough.
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22
Repeat with remaining dough.
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23
Bake, rotating sheets halfway through, until cookies are amber on top and deeply golden around the edges, 18 to 22 minutes.
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24
Transfer parchment and cookies to a wire rack to cool completely.
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25
Cookies can be kept in an airtight container at room temperature for up to 4 days.
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26
After each round of dough is brushed with egg wash, a sharp paring knife is used to score a decorative diamond pattern on the top.
