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Preheat oven to 375ºF.
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In a large bowl, mix together sugar, flour, baking powder, salt, and lemon zest.
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Using a pastry blender, cut in the butter and egg. {Dough will be crumbly.}
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Press half the dough into the bottom of a greased 9×13 inch baking dish.
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In a separate bowl, mix together the lemon juice, sugar, and cornstarch until well combined.
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Gently stir in the blueberries, just until mixed.
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Spread the blueberry mixture evenly over the crust.
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Top with the remaining dough mixture.
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Bake for 30-45 minutes or until the top of the crumbles are slightly browned.
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Let cool before cutting into squares.