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Preheat the oven to 325 F. Line 2 cookie sheets with silicone or parchment paper. Set aside.
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Place the flour, baking powder and salt into a food processor (16 cups sized bowl worked well for me, but it might have been ok in the 11 cup bowl--if you try leave a comment and let me know!). Pulse a few times to mix.
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Add the pistachios and pulse a few times to begin to chop the nuts.
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Add the well chilled butter. Pulse until it looks like bread crumbs.
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Add the sugar and pulse to mix.
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Whisk together the egg and orange blossom water in a small bowl.
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With the processor running, add the beaten egg. Stop the processor as soon as it is evenly distributed. At this point, it will not come together as a dough, but instead will look like moist crumbs.
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Clean and dry your counter. Lightly flour it. Dump the dough onto the counter, and, using your hands, gather it up and knead it a few times to form a cohesive dough (it will seem improbable but it will--and be certain not to overwork it--as soon as you have your cohesive dough, stop kneading).
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Roll the dough out to 1/4-inch thickness on a lightly floured counter. Cut out the cookies using a cookie cutter or biscuit cutter, aiming for a 2 1/2-3 inch diameter.
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Place the cookies onto the prepared cookie sheets, 1-2 inches apart (I got 12 to a sheet). Sprinkle with coarse sugar. Bake 2 sheets at a time--refrigerate the scraps until you can roll them out for a freed up cookie sheet. At the very end, after rolling out 36 cookies, I just patted out a large circle and baked it, because every time you roll out the dough the cookies will spread more.
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Bake for 20-25 minutes, rotating the 2 cookie sheets top to bottom and front to back halfway through, until the cookies are browning at the edges and turning a light golden brown.