As with most cakes that use fruit purees, this was wonderfully moist. I think most people might also call it "dense", but I've certainly eaten much denser chocolate cakes before. This one felt springy and fluffy, if not exactly light. I decided to be lazy when I made this, and not only just used a regular cake pan rather than a springform (making sure to butter and dust it with cocoa powder VERY thoroughly) but eschewed the whipped top that the original recipe calls for. To be festive, I made a little pumpkin stencil out of parchment paper and sifted the top with powdered sugar.
- Butter bottom and sides of pan and thoroughly dust with cocoa powder.
- Purée plums in a blender until very smooth (you need 1 1/2 cups; reserve any additional purée for another use). If plums are not ripe enough to form a very smooth purée, strain through a fine-mesh sieve.
- 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 purée into chocolate mixture until well combined.
- Preheat oven to 350 F.
- 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.
- Combine flour, baking powder, and salt in a medium bowl, then fold dry ingredients into chocolate mixture until combined. (You may wish to sift gradually into the mixture while folding thoroughly in between, as the dry ingredients tend to clump especially badly in this type of batter.)
- Pour batter into prepared pan and smooth surface. Bake cake until center is set and sides are beginning to brown, about 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).
- Unmold cake (run a knife around the edge to help release, if needed) and transfer to a platter.
- (Optional) For whipped cream top: Using an electric mixer on medium-low speed, beat cream in a large bowl until soft peaks form. Mound whipped cream in center of cake, then dust with cocoa powder.