In a medium-sized bowl, fold the custard and yogurt together with the seeds and juice of the passionfruit. Slice open the vanilla pod and scrape out the seeds with a knife, then stir them into the custard mixture.
Divide the mixture equally between 4 ramekins and put in the fridge to chill for 10 minutes.
Meanwhile, make the caramel. Sprinkle the sugar evenly into the base of a frying pan and warm over a low heat for about 10 minutes, until it begins to caramelise. You may need to gently move the pan around to stop it burning. Pour a thin layer of the caramel over the custard in the 4 pots, then let the caramel harden (about 2 minutes) before serving.