1.Place prunes and liqueur in a small saucepan; bring to the boil. Reduce heat; simmer, uncovered, for 2 minutes or until liqueur is reduced by half. Remove from heat; cool.
2.Meanwhile, preheat oven to 180°C/350°F. Grease a deep 22cm (9-inch) round cake pan; line base and side with baking paper.
3.Stir chocolate and butter in a small saucepan over low heat until melted and smooth. Cool to room temperature.
4.Beat egg yolks and sugar in a small bowl with an electric mixer until thick and creamy. Transfer mixture to a large bowl; fold in prune mixture, chocolate mixture and ground hazelnuts.
5.Beat egg whites in a small bowl with an electric mixer until soft peaks form; fold into chocolate mixture, in two batches. Pour mixture into pan.
6.Bake cake for 50 minutes or until a skewer inserted into the centre comes out with moist crumbs attached. Stand cake in pan for 15 minutes; turn cake, top-side up, onto a wire rack to cool.
7.To make hazelnut cream, beat cream and sugar in a small bowl with an electric mixer until firm peaks form. Stir in liqueur.
8.Dust cake with cocoa; serve with hazelnut cream.
The cake can be made a day ahead; store in an air-tight container at room temperature. Hazelnut cream is best made on the day of serving.Note