Place chocolate in a heatproof, microwave safe bowl. Microwave on medium, stirring every minute with a metal spoon, for 2-3 minutes or until smooth. Add butter. Stir until smooth. Set aside for 2 minutes. Stir in egg yolks.
Place ¾ cup cream in a bowl, using an electric mixer, beat until soft peaks form.
Beat egg whites in a bowl until soft peaks form. Gently fold cream into chocolate mixture. Stir in 2 tablespoons of the egg white. Fold in remaining egg white.
Finely chop 7 Tim Tam biscuits and fold into mousse. Spoon mousse into 1 cup- capacity glasses. Refrigerate for 3 hours or until firm.
Using an electric mixer beat remaining cream until soft peaks form. Chop remaining Tim Tam biscuits. Top mousse with whipped cream and Tim Tam just before serving.