MICROWAVE broken baking bars and 1/3 cup cream in small, uncovered, microwave-safe bowl on MEDIUM-HIGH (70%) power for 45 seconds; STIR. If pieces retain some of their original shape, microwave at additional 10- to 15-second intervals, stirring just until melted. Cool completely.
BEAT remaining 2/3 cup cream, powdered sugar and vanilla extract in small mixing bowl until stiff peaks form (do not overbeat).
FOLD melted baking bar mixture into whipped cream. Spoon into four individual dishes. Refrigerate for at least 15 minutes before serving.
NOTE: for soft-set mousse, beat cream just until soft peaks form. Mousse can also be layered with fresh berry slices.