In a small bowl, sprinkle the gelatin over the warm water. Let it sit for about 5 minutes to bloom.
Gently heat the gelatin in the microwave for 10–15 seconds or over a double boiler to dissolve it completely. Set it aside to cool slightly.
Place the white chocolate in a heatproof bowl. Heat the milk in a saucepan until it begins to steam. Pour the hot milk over the white chocolate and let it sit for 2 minutes.
Gently stir until the chocolate has fully melted and the mixture is smooth. Let it cool for a few minutes. Stir in vanilla extract and the dissolved gelatin.
In a chilled bowl, whip the heavy cream with powdered sugar (if using) until soft peaks form.
Once the white chocolate mixture has cooled to room temperature, gently fold in the whipped cream using a spatula.
Spoon the mousse into individual serving dishes or one large bowl. Chill the mousse in the refrigerator for at least 2-3 hours to allow it to set properly.
Once set, serve the mousse as-is or garnish with fresh berries, grated chocolate, or mint leaves.