How to Make It
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.