In a small heavy saucepan, combine 12 ounces of coarsely chopped white baking chocolate and 2/3 cup of heavy cream. Cook over medium-low heat until fully melted and smooth, stirring frequently.

Once melted, transfer the chocolate mixture into a large bowl. Allow it to cool at room temperature for about 15-20 minutes until no longer warm.

In a separate bowl, beat the remaining 1 1/3 cups of cold heavy cream until it begins to thicken. Add 1 tablespoon of confectioners' sugar and 1 teaspoon of vanilla extract; continue beating just until soft peaks form.

Gently fold 1/4 cup of the whipped cream into the cooled chocolate mixture to lighten it, then carefully fold in the remaining whipped cream until no white streaks remain.

Spoon the mousse into dessert dishes and refrigerate, covered, for at least two hours or until set. For an elegant touch, garnish with mixed fresh berries just before serving.