In a small bowl, combine the gelatin and water. Stir and let sit for a few minutes to allow the gelatin to hydrate.

Add the finely chopped white chocolate to a medium heatproof bowl.

In a small saucepan, heat 1/4 cup of the heavy cream over medium-high heat until it just comes to a boil.

Once it reaches a boil, remove from the heat and stir in the gelatin mixture until fully dissolved. Immediately pour the hot cream over the white chocolate. Let sit for 2–3 minutes to melt the chocolate, then stir until smooth. Transfer to the refrigerator and chill for 10–20 minutes.

While the chocolate mixture chills, whip the remaining 1/2 cup of heavy cream in a large bowl to stiff peaks. Be careful not to overwhip.

Once the chocolate mixture is chilled, remove it from the refrigerator and whisk to loosen. Add one-third of the whipped cream and whisk until mostly combined — this helps lighten the mixture.

Add the remaining two-thirds of the whipped cream and gently fold with a spatula until fully incorporated, taking care to preserve as much air as possible.

Transfer the mousse to serving dishes or leave in one bowl, then cover the surface with plastic wrap. Refrigerate for at least 4 hours, or until fully set.

To serve, spoon the mousse into bowls and top with fresh passion fruit.