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Weigh your flours; or measure them by gently spooning them into a cup, then sweeping off any excess.
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In a large bowl, whisk together the flours and salt. Place the bowl in the freezer until the flour is cold, about 10 minutes. This step ensures the flakiest final crust.
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While the flour chills, dice the cold butter into small cubes, or cut it into pats.
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Using a pastry blender, bowl scraper, or your fingertips, work the butter into the chilled flour, pressing the pieces to flatten them into flakes about the size of quarters (1" in diameter).
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Make a well in the center of the flour and butter mixture and pour in the sourdough starter, beginning with the lower amount (1/2 cup or 113g). Using a bowl scraper, mix the sourdough into the flour and butter mixture. Initially, it will be difficult to combine the ingredients; this is normal. Continue to mix the ingredients in the bowl, using pressing and folding motions, until the dough starts to become cohesive. If the dough remains dry and doesn’t come together after a few minutes, add the remaining portion of sourdough (1 generous tablespoon or 20g).
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Transfer the dough and any remaining bits in the bowl to a clean work surface and press them together with the palm of your hand.
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When most of the dry bits are incorporated, lightly flour your work surface then gently press or roll the dough into a disk, about 1/2" thick or 6" to 7" in diameter. Cover and chill the dough for at least 20 minutes.
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After chilling, roll the dough into a 13" circle and fill as desired or prebake. Alternatively, wrap the disk well and refrigerate for up 2 days or freeze for up to 2 months (thaw overnight in the refrigerator before using). If the dough has been refrigerated longer than 30 minutes, allow it to warm at room temperature for 10 to 15 minutes before rolling.