Cake surrounding creamy fruit mousse promises the best of old-world elegance. But it doesn’t matter how gorgeous it looks if the filling is rubbery and the cake is tough. The classic Charlotte Russe, a grand dessert consisting of a Bavarian cream encased in sponge cake, has fallen out of favor, partly because it’s fussy to make, and partly because lean sponge cake and bouncy gelatinized mousse don’t hold much appeal for modern cooks. In our recipe, we swap the sponge for a simple and…