Preheat the oven to 200°C.
Roast rhubarb lengths with water and brown sugar for 15 minutes and drain, reserving juices.
Preheat the oven to 220°C.
Roll the puff pastry thinner and cut into three rectangles.
Score a 1.5cm border on each rectangle and place on a greased baking sheet.
Arrange rhubarb on pastry, brush edges with reserved juice, and bake for 20 minutes.
Beat egg yolks and caster sugar, then mix in mascarpone and vanilla.
Whisk egg whites to stiff peaks and fold into the mascarpone mixture.
Serve the warm tarts with the mascarpone cream.