In a small bowl, soak the powdered gelatin in about 1/4 cup of the heavy cream. Let it sit while you bring the rest of the heavy cream and the vanilla sugar to a boil in a medium saucepan over medium heat.

Remove the pan from the heat, scrape the gelatin and cream mixture into the pan with the hot cream, and whisk until the gelatin has dissolved.

Strain the panna cotta into a serving dish and chill until set, about 3 hours.

Cook the 1/4 cup of vanilla sugar in a small saucepan over medium heat. Stir with a wooden spoon until the sugar melts and turns a medium amber color.

Reduce the heat to low and carefully stir in the passion fruit juice. Stir in the water.

Cook over medium heat for 2 minutes, then strain into a heatproof bowl. Let the caramel cool completely. Drizzle over the panna cotta right before serving.