For the syrup, mix the mango juice, lime juice and rum in a small pan, heat gently over a low temperature until it comes to a simmer, then turn off the heat and put to one side.
To make the zabaione, whisk the egg yolks, sugar, rum and a tablespoon of water in a heatproof bowl over a saucepan of simmering water until pale and thick, then set aside to cool.
Meanwhile, in a bowl, whisk the mascarpone, creme fraiche and lime zest to combine, then fold in the cooled zabaione. In a second bowl, whisk the egg whites to soft peaks, then gently fold them through the mascarpone mixture.
To assemble, briefly dip half the savoiardi in the syrup and arrange the soaked biscuits in the base of a 25cm serving dish. Neatly spoon over half the mascarpone mixture, and top first with half the sliced mangoes and then half the passion fruit pulp. Repeat with remaining savoiardi dipped in syrup, and the rest of the mangoes and passion fruit. Finally, dollop the mascarpone mixture on top, smooth out evenly and refrigerate for four to six hours.
Once chilled, scatter over the toasted coconut and the lime zest, and serve.