Preheat oven to 150 degrees C. Butter a 23 cm springform tin and line bottom with baking paper.
In the top of a double boiler, heat chocolate and unsalted butter, stirring occasionally, until chocolate is melted and smooth. Remove from heat and allow to cool to lukewarm.
In a large bowl, beat egg yolks and sugar until thick and lemon-coloured. Fold into chocolate mixture; set aside. In a separate bowl beat egg whites until stiff peaks form. Fold 1/3 of the whites into the chocolate mixture, then quickly fold in remaining whites until no streaks remain. Pour 2/3 of mixture into prepared pan. Cover and refrigerate remaining 1/3 of mousse.
Bake in the preheated oven for 40 to minutes or until filling is just firm in the centre. Allow to cool, then remove from pan and refrigerate 1 hour. Spread remaining mousse over cooled cake and refrigerate overnight.