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Preheat oven to 350 degrees. Line a 9x13 inch baking sheet with parchment paper or spray with nonstick cooking spray.
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Using a food processor, pulse sugar, baking powder, salt, and flour until mixed.
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Add cold butter slices and pulse until small pea sized clumps form.
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Add egg and ½ of lemon zest to dough and pulse until combined. Dough will be crumbly.
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If you do not have a food processor, sift together sugar, baking powder, salt, and flour. Using a pastry cutter, cut in butter to flour mixture. Add egg and half of lemon zest until you have a crumbly dough.
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Press half of dough into the pan.
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Mix blackberries with lemon juice, sugar, and cornstarch.
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Sprinkle blackberry mixture over bottom crust.
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With remaining dough, sprinkle over blackberry mixture.
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Bake for 45 minutes, or until top has slightly browned.
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Cool for 10-15 minutes then chill for 1 hour (not obligatory but recommended) before cutting into bars.