Heat oven to 140C/120C fan/gas 1. Line two large baking sheets with non-stick baking parchment. Draw eight circles, 6-8cm in diameter, on the parchment, then flip over. In a large bowl, whisk the egg whites until stiff peaks form. Gradually whisk in the sugar until thick and glossy, then the cornflour. Pile the meringue in soft swirls onto the marked circles on the baking parchment, then bake on the lowest shelf for 1 hr until crisp on the outside and dry underneath. Cool, then gently peel off the paper. Can be made up to a day in advance, just store in airtight containers.
Place the rhubarb batons in a large pan along with the sugar, vanilla and about 200ml water. Bring to a gentle simmer, then cook very gently until just soft. Discard the vanilla. Remove the rhubarb from the pan, reserving the cooking liquid. Purée half of the fruit with a hand blender or liquidiser, then strain into a bowl through a fine mesh sieve.
Reduce the rhubarb cooking liquid until thick and syrupy. Leave rhubarb and syrup to cool (can be made up to 2 days ahead and chilled until needed).
To serve, whip the cream until it just forms peaks. Carefully ripple the rhubarb purée through the cream. Plate each meringue and spoon on some of the rhubarb rippled cream. Top with the remaining batons and spoon some of the rhubarb syrup on top and around the plate.