INGREDIENTS
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1/2 stick unsalted butter, softened, plus more for pan
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3/4 cup all-purpose flour, plus more for pan
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1 cup plus 3 tablespoons quick-cooking oats, divided
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2/3 cup water
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1/2 teaspoon baking soda
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1/2 teaspoon baking powder
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3/4 teaspoon coarse salt
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1/2 teaspoon ground cinnamon
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1/2 cup granulated sugar
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1/2 cup packed light-brown sugar
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1 large egg
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1 teaspoon pure vanilla extract
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2 cups mixed ripe or overripe blueberries and blackberries, divided
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3 tablespoons coarse sanding sugar