INGREDIENTS
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6 tablespoons (3/4 stick) unsalted butter, at room temperature, plus extra for greasing the dish
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10 to 12 purple "prune" plums, cut in half and pitted
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1 3/4 cups granulated sugar, divided
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2 extra-large eggs, at room temperature
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1/3 cup sour cream
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1/2 teaspoon grated lemon zest
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1/2 teaspoon pure vanilla extract
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1 cup plus 2 tablespoons all-purpose flour
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1/2 teaspoon baking powder
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1/4 teaspoon kosher salt
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Confectioners' sugar