Preheat oven to 150 degrees C. Butter a 20cm springform pan, 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 1/4 cup sugar until thick and lemon-colored. Fold into chocolate mixture; set aside. In a large glass or metal mixing bowl, beat egg whites until foamy. Gradually add 1/8 cup sugar, continuing to beat 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.
Bake in the preheated oven for 40 minutes, or until filling is just firm in the centre. Allow to cool, then remove from pan and refrigerate 1 hour.
In a medium bowl, whip cream to soft peaks and fold into remaining chocolate mixture. Cover and refrigerate.
Spread remaining mousse over cooled cake and dust with cocoa.