1.Oil deep 19cm-square cake pan with oil. Make jelly according to packet directions; pour into pan. Refrigerate 3 hours or until set.
2.Make white chocolate ganache. Stir cream and chocolate in medium heatproof bowl over medium saucepan of simmering water until smooth (do not let water touch base of bowl). Cool.
3.Pour three-quarters of ganache over the jelly; refrigerate 1 hour.
4.Meanwhile, preheat oven to 180°C/160°C fan-forced. Grease 23cm-square cake pan; line base with baking paper.
5.Beat eggs in small bowl with electric mixer until thick and creamy. Gradually beat in sugar, beating until sugar dissolves. Fold in triple-sifted cornflour. Spread mixture into pan.
6.Bake cake about 20 minutes. Turn cake onto baking-paper-covered wire rack to cool.
7.Trim cake to 19cm-square; place in pan on top of ganache and jelly. Refrigerate 30 minutes.
8.Meanwhile, spread chocolate onto baking-paper-lined tray until 3mm thick; refrigerate about 10 minutes or until set. Break into small pieces.
9.Place base of pan in sink of hot water for a few seconds to loosen jelly; quickly invert cake onto serving plate. Secure chocolate pieces around edges of cake with remaining ganache; secure ribbon.