In a small, heavy-bottomed saucepan, place the white chocolate together with 2/3 cup of the heavy cream. Warm over medium-low heat, stirring continuously, until the chocolate is fully melted and the mixture is completely smooth. Transfer this mixture to a large bowl and let it cool to room temperature.

Pour the remaining heavy cream into a small bowl and beat until it starts to thicken. Add the powdered sugar and vanilla extract gradually, then continue beating until soft peaks form. The cream is ready when the peaks curl over gently rather than holding a firm shape.

Fold 1/4 cup of the whipped cream into the cooled white chocolate mixture to lighten its texture. Once blended, gently incorporate the rest of the whipped cream in portions, taking care not to stir too vigorously so the mousse remains light.

Spoon the finished mixture into individual serving dishes. Cover and refrigerate for at least 2 hours so the mousse can set properly and the flavor can fully develop.

Just before serving, top with fresh berries if desired. Serve chilled.