INGREDIENTS
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1/2 cup unsalted butter, room temperature, plus more for baking dish
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2 cups all-purpose flour
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1/2 teaspoon baking soda
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1/2 teaspoon salt
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1 cup buttermilk
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1 teaspoon pure vanilla extract
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1 1/4 cups sugar
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1 large egg
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2 cups chopped rhubarb
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1/2 teaspoon ground cinnamon