1.Have eggs at room temperature.
2.Preheat oven to 180°C (160°C fan-forced). Grease deep 22cm (9-inch) square cake pan; line base and sides with baking paper, extending paper 5cm (2 inches) over sides.
3.Break dark chocolate into small saucepan, add the water; stir over low heat until smooth. Cool to just warm.
4.Separate eggs. Beat egg yolks and sugar in small bowl with electric mixer about 5 minutes or until pale and thick. Beat in chocolate mixture until combined. Transfer mixture to large bowl.
5.Beat egg whites in small bowl with electric mixer until soft peaks form. Fold egg whites into chocolate mixture, in two batches. Pour mixture into pan; smooth surface. Bake about 17 minutes; cool sponge in pan.
6.Place ice-cream in medium bowl; beat a with wooden spoon until barely softened. Spread ice-cream over sponge in pan; smooth surface. Cover with foil; freeze overnight.
7.Break milk chocolate into medium heatproof bowl, place over medium saucepan of simmering water (don’t let water touch base of bowl); stir until smooth. Remove from heat; cool to just warm.
8.Meanwhile, beat cream in small bowl with electric mixer until firm peaks form; refrigerate until ready to use. Whisk slightly warm chocolate mixture into cream. Pour over ice-cream layer; smooth surface. Freeze overnight.
9.Finely chop extra dark chocolate; combine with sifted cocoa in medium bowl. Sprinkle chocolate mixture over dessert just before serving.
To cut this dessert, use a large hot dry kitchen knife. Cut as many portions as you need, and freeze any remaining dessert covered with foil. Cut the dessert as soon as you take it out of the freezer, but wait about 10 minutes before serving.Note