Directions
Preparation:45min  ›  Cook:40min  ›  Extra time:2hours chilling  ›  Ready in:3hours25min 

1. Preheat oven to 150 degrees C. Butter a 20cm springform pan, and line bottom with baking paper.
2. 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.
3. 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.
4. 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.
5. In a medium bowl, whip cream to soft peaks and fold into remaining chocolate mixture. Cover and refrigerate.
6. Spread remaining mousse over cooled cake and dust with cocoa.