Place rhubarb, 50g sugar, and water in a covered pan.
Cook over medium heat until rhubarb softens and breaks down.
Whisk egg yolks and remaining sugar until pale.
Simmer milk, cream, and vanilla, then remove the pod.
Whisk hot milk into the egg mixture until smooth.
Thicken the custard over medium heat until it coats a spoon.
Stir in 75% of the rhubarb and churn in an ice cream maker for 45 minutes.
Transfer to a container when almost solid.
Ripple in remaining rhubarb and freeze for 4 hours.
Preheat oven to 200°C.
Roast rhubarb with cider, syrup, five-spice, and vanilla for 10 minutes.
Remove rhubarb from liquid and set aside to cool.
Boil roasting liquid for 3 minutes until thickened into a sauce.
Increase oven temperature to 220°C.
Boil milk and vanilla pod for the tart custard.
Whisk sugar and egg yolks until fluffy.
Combine hot milk with the egg mixture.
Return custard to the pan and simmer.
Whisk in cornflour until thick and cool with cling film on top.
Roll pastry to 0.5cm thick and cut four 15cm discs.
Spoon custard onto pastry discs leaving a 1cm border.
Place roasted rhubarb stems on top of the custard.
Bake for 15 minutes until pastry is golden-brown.
Serve tart with 2 scoops of ice cream and cider sauce.