Process biscuits until fine. Transfer to a large bowl; stir in butter. Press the biscuit mixture evenly over base and 2/3 of the way up the side of a 24cm springform pan; refrigerate for about 30 minutes or until firm.
Preheat oven to 150°C (130°C fan-forced). Place the cream and chocolate in a small, heavy-based saucepan; stir over a low heat until smooth. Cool for 5 minutes.
Beat the cream cheese and caster sugar in a medium bowl with an electric mixer until smooth; add mascarpone, beat until just combined. Beat in eggs, one at a time, until just combined; fold in chocolate mixture and cornflour.
Place pan on an oven tray; pour cheese cake mixture into the pan. Gently press 150g of the raspberries onto the surface. Bake for about 50 minutes. Cool in oven with door ajar. Refrigerate for 3 hours or overnight. Decorate with remaining raspberries and dust with a little sifted icing sugar, if desired.