Step 1 Grease a 6cm-deep, 22cm (base measurement) round spring form pan. Line base and sides with baking paper. Place 1 packet of Tim Tam in a food processor, process until finely chopped.
Step 2 Add butter, process until combined. Press mixture evenly over base of prepared pan. Refrigerate for 30 minutes.
Step 3 To make chocolate mousse; place chocolate and half the cream in a saucepan over low-medium heat. Cook, stirring constantly for 3-4 minutes or until smooth. Remove from heat. Cool for 15 minutes.
Step 4 Using an electric mixer, beat remaining cream until soft peaks form. Fold chocolate mixture and ½ a pack of the remaining Tim Tam biscuits into cream. Pour over Tim Tam base. Chill overnight.
Step 5 Garnish with remaining ½ pack of Tim Tam in the center of the cake and whipped cream to serve.