Put rhubarb and 4 tablespoons sugar in a baking dish with 3 tablespoons water. Cover with foil and bake at 180c until rhubarb is tender, around 10-15 minutes. Leave to cool.
Put milk on to the heat until just below boiling. While heating, whisk together egg yolks, cornflour, 1 tablespoon caster and vanilla paste. Once milk is just bubbling, pour slowly over the yolk mixture and continue whisking to incorporate.
Pop back into the pan and whisk over low heat until it thickens. Leave to cool.
Whisk double cream until soft peaks form. Fold custard through gently and then add rhubarb and combine to a ripple effect.
Drizzle over some of the cooking liquor from the rhubarb and crumble ginger nuts over the top if wished.
