1. Cook rhubarb, water and sugar over medium low heat until the mixture thickens and the rhubarb is cooked through (enough so you can pierce it with a fork easily).
2. For the custard, bring sugar and whipping cream to a boil in a saucepan. Pour slowly over egg yolks while whisking constantly. Strain, then add lemon zest.
3. Spoon rhubarb mixture into ramekins evenly. Pour over the custard to just about full.
4. Bake custard at 150° for about 20-25 minutes until set.
5. When cool, crumble hobnobs on top and drizzle with melted white chocolate.