INGREDIENTS
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1 stick unsalted butter, softened, plus more for the baking dish
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4 cups blackberries
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4 cups blueberries
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1/2 cup maple syrup
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1/3 cup instant tapioca
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1 1/4 cups all-purpose flour
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1/2 cup granulated sugar
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1/3 cup packed light brown sugar
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1/2 teaspoon freshly grated nutmeg
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Pinch of salt
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2/3 cup blanched hazelnuts
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1 teaspoon vanilla extract