- 1 cup pitted prunes
- 3/4 cup unsalted butter (1 1/2 sticks), plus more for the pan
- cocoa powder, for the pan
- 6 ounces bittersweet chocolate, chopped
- 6 ounces semisweet chocolate, chopped
- 6 large eggs, separated
- 1 teaspoon pure vanilla extract
- 1/2 teaspoon fine salt
- 3 tablespoons sugar
- Soak the prunes in boiling water until very soft, 20 to 30 minutes. Heat oven to 325° F. Butter a 9-inch springform pan and dust with cocoa, tapping out the excess.
- Drain the prunes and transfer to a food processor. Process until finely chopped, 10 to 15 seconds.
- Place the chocolate and butter in a large microwave-safe bowl. Microwave on high in 30-second intervals, stirring in between each, until melted and smooth. Add the prunes, egg yolks, vanilla, and salt and whisk to combine.
- Beat the egg whites in a large bowl with an electric mixer on medium-high until soft peak forms, 2 to 3 minutes. Continue beating, slowly adding the sugar, until stiff peak forms, 1 to 2 minutes more. Gently fold the egg whites into the chocolate mixture.
- Pour the mixture into the prepared pan. Bake until the cake is set around the edges but still slightly soft in the middle, 40 to 45 minutes. Let cool.