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1
Position a rack in the center of the oven and heat to 350F.
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2
Butter and flour a round 9-inch-wide by 2 1/2-inch-tall cake pan and line the bottom with parchment.
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3
Split the vanilla bean lengthwise and scrape the seeds from the pod.
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4
Transfer the seeds to a medium bowl and add the flour, baking powder, and salt.
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5
(To use the pod, place it in a jar and cover with sugar.
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6
After infusing for a month, the sugar will take on the vanilla flavor.)
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7
Whisk to blend (use your fingers to blend in the vanilla seeds) and set aside.
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8
Combine the almond paste and sugar in the bowl of a stand mixer fitted with the paddle attachment.
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9
Beat on low speed until the almond paste breaks apart and starts to resemble fresh breadcrumbs, 1 to 2 minutes.
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10
Add the butter, increase the mixer speed to medium, and continue to mix until the crumbs start to aggregate into larger clumps, 2 minutes longer.
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11
Increase the speed to medium-high and with the motor running, add the eggs, one at a time, beating well after each addition and scraping the bowl as necessary.
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12
Add the vanilla extract and mix briefly to blend.
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13
Add the flour mixture and mix on low speed until the batter just comes together, about 15 seconds.
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14
Remove the bowl from the mixer and gently fold in the plums.
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15
Pour the batter into the pan and smooth the top.
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16
Bake until the top is dark golden brown and a skewer inserted into the center comes out clean, 65 to 70 minutes.
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17
Let cool in the pan for 15 minutes, and then turn out onto a cooling rack.
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18
Let cool completely before serving.
