Heat cream and vanilla seeds in a saucepan over medium heat until hot (not boiling), then remove from heat.
Combine one egg and cornflour and beat with a fork to form a paste. Add remaining egg yolks, caster sugar and whisk until thick and pale.
Whisk into hot cream mixture. Place in a heatproof bowl and whisk over simmering water until mixture thickens and coats the back of a spoon.
Cover with plastic wrap and cool in fridge. Fold extra whipped cream through custard.
Sponge Assembly
Combine juice and Cointreau in shallow bowl. Dip all the biscuits in juice mixture then place biscuits in a single layer over the base of a 3 litre glass bowl.
Top biscuits with half of the mango and passionfruit pulp.
Spread half the custard over the top.
Top with remaining biscuits and repeat layer of fruit and custard.
Cover custard with plastic wrap and refrigerate overnight.
Decorate with mango and blueberries before serving.
Notes
Substitute other fruit in season or canned fruit if desired.