INGREDIENTS
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1 tablespoon sour cream
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1 tablespoon fresh lemon juice
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2 tablespoons ice water, (plus extra if needed)
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1 1/4 cups unbleached all-purpose flour, (plus more for rolling)
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1/2 teaspoon coarse sea salt, (divided)
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1/3 cup plus 1 teaspoon granulated sugar, (divided)
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8 tablespoons cold unsalted butter, (cut into small pieces)
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3/4 cup fresh blackberries
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3/4 cup fresh raspberries
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4 1/2 teaspoons arrowroot ((or cornstarch))
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1 large egg, (lightly beaten)
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Sanding Sugar or Turbinado sugar for sprinkling