1.Preheat oven to 180°C (160°C fan-forced). Grease 25cm x 30cm swiss roll pan; line base and sides with baking paper.
2.Beat egg yolks and sugar in small bowl with electric mixer until thick and creamy; transfer to large bowl. Fold in combined sifted cocoa and cornflour, then chocolate; fold in the water.
3.Beat egg whites in medium bowl with electric mixer until soft peaks form. Fold egg whites, in two batches, into chocolate mixture. Spread mixture into pan; bake about 15 minutes. Turn cake onto baking paper covered wire rack. Cover cake with baking paper; cool to room temperature.
4.Grease 22cm springform tin; line side with baking paper, bringing paper 5cm above edge of tin. Cut 22cm-diameter circle from cooled cake; place in tin. Discard remaining cake.
5.Stir cream and extra chocolate in medium saucepan over low heat until smooth; transfer to large bowl. Refrigerate until just cold.
6.Add liqueur to chocolate mixture; beat with electric mixer until mixture changes to a paler colour. Pour mixture into tin; refrigerate about 3 hours or until set.
7.Transfer cake from tin to serving plate; dust with sifted extra cocoa.
Do not overbeat the chocolate and liqueur mixture as it will curdleNote