1.To make crème pâtissière, split vanilla bean lengthways and scrape seeds into a small saucepan. Discard bean. Add milk and sugar to pan and bring to the boil. Meanwhile, combine cornflour and egg yolks in a medium heatproof bowl; gradually whisk in hot milk mixture. Return custard to pan; stir over heat until mixture boils and thickens. Cover surface of custard with plastic wrap; cool to room temperature. Fold cream into custard.
2.Preheat oven to 180°C. Grease a 12-hole oval friand pan with butter; dust lightly with a little extra flour.
3.Beat eggs and sugar in small bowl with electric mixer about 5 minutes or until thick and creamy. Transfer mixture to large bowl and fold in sifted flour. Combine 20g of the butter and boiling water; add to mixture and fold through. Divide mixture into pan holes.
4.Bake cakes about 15 minutes. Working quickly, use a small knife to loosen edges of cakes from pan; turn immediately onto baking-paper-covered wire rack to cool.
5.Stir chocolate and remaining butter in small heatproof bowl over small saucepan of simmering water until smooth.
6.Split cold cakes in half, sandwich with crème pâtissière. Top with chocolate mixture.