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1
Preheat oven to 350u00b0F. Butter bottom and sides of springform pan and line bottom with parchment.
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2
Puree plums in a blender until very smooth (you need 1 1/2 cups; reserve any additional puree for another use). If plums are not ripe enough to form a very smooth puree, strain through a fine-mesh sieve.
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3
Combine chocolate and 3/4 cup butter in a large heatproof bowl. Set bowl over a medium saucepan of barely simmering water (do not let bowl touch water). Stir until chocolate is melted and smooth. Remove bowl from saucepan. Stir plum puree into chocolate mixture until well combined.
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4
Using an electric mixer on medium speed, beat eggs and sugar in another large bowl until pale and tripled in volume, 10-12 minutes. Using a rubber spatula, fold about 1/3 of the egg mixture into chocolate mixture to lighten, then fold chocolate mixture into remaining egg mixture until combined.
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5
Combine flour, baking powder, and salt in a medium bowl, then fold dry ingredients into chocolate mixture until combined.
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6
Pour batter into prepared pan and smooth surface. Bake cake until center is set and sides are beginning to brown, 45-55 minutes. Transfer to a wire rack and let cake cool completely in pan (cake will sink a little in the center as it cools).
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7
Meanwhile, using an electric mixer on medium-low speed, beat cream in a large bowl until soft peaks form. Unmold cake (run a knife around the edge to help release, if needed) and transfer to a platter. Mound whipped cream in center of cake, then dust with cocoa powder.
