For the syrup, cut the rhubarb into small pieces and boil with water and sugar for 10 minutes. Purée everything using a handheld mixer or a blender, and sieve through a coffee filter. (I used a regular metal sieve at first, but it allowed some of the fruit to pass through and that made the syrup cloudy.) This will take a while - you should count on leaving it for a few hours.
For the mousse, you need 200 ml of the rhubarb syrup, and 1,5 gelatin sheets. Soak the gelatin in cold water for five minutes. Whisk the egg yolks and the sugar until fluffy and pale. Gently heat the rhubarb syrup, and add the gelatin. Stir until dissolved. Mix this with the sugar and eggs, and leave to cool to room temperature. Whisk the cream until it forms stiff peaks, and fold into the rhubarb mixture. Pour into glasses or bowls, and place in the fridge to set.
For the jelly, you need 100 ml of rhurbarb syrup, and the remaining 1 1/2 sheets of gelatin. Soak the gelatin in cold water for five minutes. Gently heat the rhubarb syrup, and add the gelatin. Stir until dissolved. Leave to cool to room temperature, then gently pour over the set mousse. Place in the fridge to set.
Just before serving, decorate with a few curls of white chocolate.