Preheat oven to 180°C. Grease a deep 22cm (8-inch) round cake pan; line base with baking paper.

Melt chocolate in a small heatproof bowl over a small saucepan of simmering water (do not allow water to touch base of bowl); stir in the water; cool mixture to room temperature.

Beat butter and sugar in a small bowl with an electric mixer until light and fluffy. Add egg yolks, one at a time, beating until combined. Transfer mixture to a large bowl; stir in chocolate mixture, then sifted flour.

Beat egg whites in a small bowl until soft peaks form, gradually beat in extra sugar, beating until dissolved between each addition; fold into chocolate mixture.

Spread mixture into pan. Bake about 30 minutes. Stand cake in pan 5 minutes before turning onto a wire rack to cool; leave cake upside-down

Meanwhile, make chocolate icing: Melt chocolate and butter in a small heatproof bowl over a small saucepan of simmering water (do not allow water to touch base of bowl). Cool at room temperature until spreadable, stirring occasionally; this can take up to 2 hours.

Split cold cake in half; place one half, cut-side up, on serving plate. Brush half the warmed jam over cake half, top with remaining cake half. Brush cake all over with remaining jam. Stand about 1 hour at room temperature or until jam has set. Spread top and side of cake with icing; stand at room temperature until icing has set.