1.Preheat oven to 170ºC/325°F. Grease a 22cm (9-inch) springform cake pan; line with baking paper.
2.Place chocolate and butter in a small saucepan over low heat; stir until smooth. Cool.
3.Beat egg yolks and ½ cup of the caster sugar in a small bowl with an electric mixer until thick and creamy. Beat in chocolate mixture. Transfer to a large bowl.
4.Beat egg whites and remaining caster sugar in a clean large bowl with an electric mixer until soft peaks form; fold into chocolate mixture, in two batches. Spoon mixture into pan.
5.Bake cake about 35 minutes. Cool in pan.
6.Meanwhile, make chocolate mousse. Break chocolate into a small saucepan; stir over low heat until smooth. Cool. Dissolve coffee in the water; cool. Beat cream in a large bowl with an electric mixer until soft peaks just form. Fold in chocolate and coffee mixture until just combined.
7.Pour mousse over cake in pan. Cover; refrigerate 6 hours or until firm. Just before serving, top with berries, dust with sifted icing sugar.
You can use a 600ml carton of cream for this recipe. This cake can be made a day ahead.Note