Mix the cornstarch with 4 tablespoon passion fruit nectar until smooth. Bring the rest of the passion fruit nectar to the boil with sugar. Stir in the mixed cornstarch and simmer for about 1 minute. Let cool down, stirring several times.
Whip the cream until stiff, drizzling in the vanilla sugar. Mix the passion fruit pudding with the yoghurt until smooth, then fold in the cream - leave about 3 tablespoon of it for the decoration. Pour the cream into three dessert glasses and chill for about 1 hour.
Garnish with the sliced nectarine, the rest of the cream and lemon balm just before serving.