Makes one 9-inch cake
For the plum caramel:
3/4 pound ripe plums
1 tablespoon freshly squeezed lemon juice
1/2 vanilla bean
3/4 cup sugar
For the cake:
2 3/4 cups all-purpose flour
1 cup sugar, plus 1 tablespoon for sprinkling
1/4 cup semolina flour
1 1/2 teaspoons baking powder
1/2 teaspoon baking soda
1/2 teaspoon kosher salt
18 tablespoons (2 1/4 sticks) unsalted butter, softened, at room temperature, plus a little for greasing the pan
1 cup crème fraîche, divided
6 egg yolks
3/4 pound ripe plums
To make the caramel: Pit the plums and cut them into small wedges. Place them in a blender with the lemon juice and purée until smooth and liquidy. If you are using ripe, juicy plums, you won't need to add any more liquid to make the purée a pourable consistency.
Use a paring knife to split the vanilla bean in half lengthwise. Scrape the seeds and pulp into a medium saucepan. Add the vanilla bean pod, the sugar, and 1/2 cup water.
Bring to a boil over medium heat, without stirring. Cook for about 10 minutes, swirling occasionally, until the mixture is a deep amber color. Slowly add the plum purée, whisking constantly. Continue cooking until any sugar that has seized is dissolved. Turn off the heat and let cool. Makes 1 cup plum caramel.
Preheat the oven to 375° F. Butter a 9-inch round springform pan, line the bottom with parchment paper, and butter the parchment.
In the bowl of a stand mixer fitted with a paddle attachment, combine the flour, 1 cup sugar, semolina, baking powder, baking soda, and salt. Paddle just to combine.
Add the butter and 1/2 cup crème fraîche and paddle until the mixture starts to come together but is still crumbly.
Add the remaining 1/2 cup crème fraîche and the egg yolks, then increase the speed to medium-high and paddle until the color lightens to a pale yellow.
Remove the pits from the plums and cut them into 3/4-inch wedges.
Spread half the batter into the prepared pan. Drizzle 1/3 cup plum caramel over the batter, then arrange half the plum wedges on top.
Dot the remaining half of the batter on top of the arranged plums and caramel, then gently spread it to cover the plums. Arrange the rest of the plum wedges on top, then sprinkle with the final 1 tablespoon sugar.
Put the cake in the oven with a sheet tray on the rack underneath to capture any caramel drips. Bake for 30 minutes, then reduce the oven temperature to 350° F and bake for 30 more minutes, until the cake begins to pull away from the sides of the pan and a toothpick inserted into the center comes out clean. Cool the cake on a rack for at least 30 minutes before unmolding.
Serve the cake drizzled with additional plum caramel.
Formulation provided by Sarah Jampel, courtesy of Food 52.