Have butter and eggs at room temperature.

Preheat oven to 170°C (150°C fan forced). Grease and lightly flour 26cm (10½-inch) closed springform pan.

Make custard. Halve vanilla bean, scrape seeds into medium saucepan, add pod. Add milk, bring to the boil, remove from heat. Whisk yolks, sugar and cornflour together in medium heatproof bowl, whisk milk mixture into egg mixture until smooth. Strain into medium saucepan; discard vanilla bean. Whisk over high heat until mixture boils and thickens. Remove from heat, whisk in rum.

Beat butter, rind and sugar in large bowl with electric mixer until smooth. Beat in eggs one at a time, do not over beat. Stir in sifted flour in two batches, mix to a spreadable dough.

Reserve a quarter of the dough. Spread remaining dough over the base and halfway up sides of pan. Spread custard evenly over dough, top with prunes.

Dollop tablespoons of the reserved dough over prunes, smooth surface with a spatula.

Whisk egg yolk with about 2 teaspoons water in small bowl, brush dough with egg mixture. Lightly scrape top of dough with a fork in a criss-cross pattern. Bake cake about 50 minutes. Cool cake in pan.