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Grease and flour a 10-inch springform pan.
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Preheat the oven to 350°F
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Halve and de-pit the plums. Cut them lengthwise into strips.
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Whisk the flour, baking powder, and salt together.
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Beat the butter, peanut butter, and sugar until thick and creamy.
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Beat in the vanilla.
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Add the eggs one at a time. Make sure each is incorporated before adding the next.
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Beat in half the dry ingredients, then the milk, then the rest of the dry ingredients.
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Transfer the batter to the prepared pan. It will be thick and sticky. Wet a spatula or your hands. This makes it easier to spread out in the pan.
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Spread the plums on top and press them into the batter.
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Bake for 30 minutes or until a knife inserted into the cake comes out clean.
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Remove from the oven, let cool.