Scoop the seeds out of the passion fruits. Set aside 1 tbsp and transfer the rest to a food processor; pulse to break down the seeds and release the juice. Sieve the pulp over a measuring jug, pressing down with a spoon or spatula to extract all the juice. Scrape any juice from the underside of the sieve and discard the pulp. Add enough lime juice to make the juice up to 50ml, if needed.
Heat the cream and sugar in a large saucepan over a medium heat until the sugar dissolves. Bring to a simmer and cook for 1 min, stirring constantly as it bubbles to stop it sticking. Remove from the heat and stir in the passion fruit juice; the mixture should thicken. Press the mixture through a sieve into a jug and stir through half the lime zest. Divide between 4 shallow glasses and leave to cool to room temperature. Cover with clingfilm and transfer to the fridge. Chill for at least 3 hrs, or up to 24 hrs.
Remove the glasses from the fridge 5 mins before serving and top with the reserved passion fruit seeds and remaining lime zest.
