Grease a 6cm-deep, 22cm diameter circular spring form pan. Line base and sides with baking paper. Place 1 packet of Tim tam biscuits in a food processor, process until finely chopped. Add butter, process until combined. Press mixture evenly over base of prepared pan. Refrigerate for 30 minutes.
Meanwhile make chocolate mousse: place chocolate and half the cream in a saucepan over medium- low heat. Cook, stirring constantly, for 3-4 minutes or until smooth. Remove from heat. Cool for 15 minutes. Using an electric mixer, beat remaining cream until soft peaks form. Fold chocolate mixture and half a pack of the remaining Tim Tam biscuits into cream. Pour over Tim Tam base. Chill overnight.
Garnish with remaining ½ pack of Tim tams in the center of the cake and whipped cream to serve.