Grease 4 x 12cm loose-bottomed tart tins with butter. Dust a work surface with flour.
Roll the pastry out to about 5mm thick. Line each tin with the pastry, easing it into the sides. Prick base of each tart with a fork a couple of times, then refrigerate for 30 minutes.
Preheat the oven to 180ºC/gas 4.
Bake the pastry cases for 15 to 20 minutes, or until golden brown. Allow to cool.
Finely chop the ginger, finely grate the orange zest and put aside, then cut the rhubarb into 5cm pieces.
Place the rhubarb in a frying pan, squeeze over half the orange juice, then add the sugar and ginger. Simmer gently for 12 to 15 minutes till softened and syrupy. Let cool, then chill until needed.
To make the custard, heat the milk in a pan to just below boiling. Beat the egg yolks and sugar in a large bowl. Mix in a little hot milk, then the cornflour. Mix in the remaining milk.
Rinse out milk pan, then return the custard to the pan and bring slowly to the boil, stirring continuously. Boil for 1 minute, stirring, to thicken, then pass through a sieve into a bowl. Place damp greaseproof paper directly on the surface to prevent a skin forming and allow to cool.
Lightly whip the cream. Once cool, fold in the cream, vanilla extract and reserved orange zest.
Spoon custard into each tart case, top with rhubarb and drizzle over some juices.