How to Make It
Line a 9-inch-round cake pan with plastic wrap and transfer it to the freezer. In a bowl, rub the sugar with the blood orange zest, lemon zest and salt.
In a medium saucepan, whisk the water with the agar and cook over moderate heat, whisking, just until slightly thickened, 3 to 5 minutes. Add the citrus sugar, blood orange juice and lemon juice and cook over moderate heat, whisking, just until the sugar is dissolves, about 5 minutes. Whisk in the coconut and almond milks and let cool completely, then refrigerate until well chilled, about 3 hours.
Strain the sherbet base through a fine sieve into the bowl of an ice cream maker and freeze according to the manufacturer’s instructions. Scrape the sherbet into the prepared cake pan and spread it in an even layer. Press a piece of plastic directly on the surface and freeze until very firm, at least 4 hours or overnight.
In a food processor, pulse the ladyfingers until very finely ground. Transfer the crumbs to a large bowl.
In the food processor, combine the prunes with the hot water, cocoa powder, maple syrup, lemon zest, vanilla and salt and pulse until smooth, scraping down the sides as needed. Scrape the mixture into the large bowl, and using your hands, knead the mixture until an even smooth dough forms. Transfer the dough onto a large piece of plastic wrap and press it into a 9-inch disk. Wrap the disk and refrigerate until well chilled, about 45 minutes.
Unwrap the dough and carefully slide it over the frozen sherbet, pressing it to cover the entire surface. Wrap the cake and freeze until firm, at least 1 hour. Invert the cake onto a plate so the crust is on the bottom. Garnish the top with thinly sliced blood orange, cut into wedges and serve.
Agar powder is a colorless type of gelatin made from seaweed that sets without chilling. It is available at some health food shops, Asian markets and from amazon.com.