Cake: Prepare and bake cake mix according to packet directions for 2 x 23cm round cake tins. Cool cakes completely, then split each in half to make 4 layers.
Raspberry Filling: Puree raspberries in a blender or food processor. In a small saucepan, heat pureed raspberries until boiling. Mix cornflour with a small amount of water and stir into raspberries. Cook, stirring constantly for 5 minutes or until thickened. Set aside to cool.
White Chocolate Mousse: In the top of a double boiler over simmering water, heat white chocolate with 1 cup of cream, stirring occasionally, until chocolate is melted and smooth. Remove from heat and allow to cool to lukewarm.
Whip the remaining 2 cups of 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.
To Assemble: Place 1 cake layer on a serving plate and spread with 1/4 of the white chocolate mousse. Place the next layer on top and spread with raspberry filling. Repeat next layer of cake with 1/4 of the mousse. Cover with the final layer of cake. Cover top and sides with mousse and pipe a border with remaining mousse. Garnish with fresh berries.