Preheat the oven to 170°C.
Rub the butter and flour together then mix in the sugar and egg until dough forms.
Compress the dough into a flat circle and chill for one hour.
Roll out the pastry to 5mm thickness and line four tart cases.
Blind bake the cases with parchment and beans for 15-20 minutes until golden.
Boil the cream with vanilla seeds and pods in a saucepan.
Whisk sugar and eggs together then gradually pour in the warm cream.
Cook the custard until thick enough to coat a spoon then pass through a sieve.
Fill the tart cases halfway with custard and chill for two hours.
Preheat the oven to 200°C.
Bake the rhubarb with grenadine, sugar, and orange juice for five minutes.
Cut cooled rhubarb into 1cm pieces and drain while reserving the juices.
Arrange the rhubarb pieces on top of the set custard.
Reduce marmalade and rhubarb juices to a syrup and glaze the tarts.
Serve with clotted cream and more glaze.