Soften the gelatine leaves in cold water. Halve the passion fruit and remove the seeds, working over a bowl to catch as much juice as you can. Reserve the seeds for decoration.
Combine the crème fraîche, yogurt and passion fruit juice.
Put 100 ml (3½ fl oz) water in a small saucepan, add the sugar and the seeds from the vanilla pod and heat gently, stirring until the sugar has dissolved. Drain the gelatine and add to the pan. Stir until dissolved, then leave to cool to room temperature.
Mix the gelatine mixture into the crème fraîche, then pour into 4 ramekins or moulds. Refrigerate for 6 hours or until set.
Turn the panna cotta out of their moulds by briefly immersing each ramekin in very hot water. Spoon over the reserved seeds to decorate.