Prepare and bake cake mix according to package directions for two 9 inch round pans. Cool cakes completely, then split each in half to make 4 layers.
To make Raspberry Filling: In a blender or food processor, puree raspberries. In a saucepan, heat pureed raspberries until boiling. Mix cornstarch with a small amount of water and stir into raspberries. cook, stirring constantly, for 5 minutes, or until thickened. Set aside to cool.
To make White Chocolate Mousse: In the top of a double boiler, heat white chocolate with 1 cup of the cream, stirring occasionally, until chocolate is melted and smooth. Remove from heat and allow to cool to lukewarm. Whip the remaining 2 cups cream until soft peaks form. Stir in the vanilla. Fold 1/3 of the whipped cream into the white chocolate mixture, then quickly fold in the remaining whipped cream. Do not over-mix, or the mousse will become grainy.
Place 1 cake layer on serving plate and spread with white chocolate mousse. Place the next layer on top and spread with raspberry filling. Repeat next layer of cake with mousse. Cover with the final layer of cake. Frost top and sides with mousse. Pipe a border with remaining mousse. Garnish with fresh berries.