To make the mousse, place the sugar and water in a large saucepan and boil for approximately 6-7 minutes until the mixture takes on a syrupy consistency, but do not allow it to take on a brown colour. Take the saucepan off the heat and set to one side for a moment.
Beat the egg whites until stiff, then whisk continuously as you add them to the sugar syrup. Set to one side.
Cut the passion fruits in half and scrape the seeds and juice into a large bowl.
Place the warm water in a bowl and sprinkle the gelatine on top. Place the bowl in a pan of hot water for 2-3 minutes until the gelatine dissolves. Remove the bowl from the pan.
Stir a spoonful of the passion fruit into the gelatine. Add this to the rest of the passion fruit, then add the cream and/or yoghurt and mix well. Finally, add in the egg-white syrup. Pour the mixture into four individual moulds, approximately 5-7.5cm / 2-3inches in diameter, and chill in the fridge for at least 1 hour until set and firm.
To make the syrup, place the water and sugar in a large saucepan. Heat, stirring continuously, until all the sugar has dissolved. Mix the arrowroot with the water, and add it along with the lime juice, to the saucepan. Bring to the boil and simmer for 1 minute.
To assemble the dessert, unmould the individual servings of passion fruit mousse and place on a dessert plate. They should come out easily but if you are finding it difficult simply place the moulds in a basin of warm water for 30 seconds, then turn upside down onto the serving plate and the moulds should come away easily. Drizzle the mousse generously with tangy lime syrup and serve.