The additions of crème fraîche, semolina flour, and thick fruit slices are not what make this cake-which comes from The A.O.C. Cookbook by Suzanne Goin-special. No, its real signature is the "plum caramel," which is made by puréeing plums (this will feel like a crime, so close your eyes) with lemon juice and then whisking this mixture into caramelized sugar.