Roughly slice up the rhubarb and place it in a pan with the caster sugar and 5 tablespoons of water. Simmer until tender, divide between 6 small serving dishes which your brulee will be cooked in, then set aside.
Score the vanilla pods lengthwise and run the knife up the pod to remove the vanilla seeds. Scrape these into the pan with the pods, cream and milk and slowly bring to the boil.
Meanwhile beat together the yolks and the sugar in a bowl until light and fluffy.
When the cream and milk are just boiling, remove the vanilla pods and add little by little to the egg mixture, whisking continuously.
I like to remove any bubbles or froth from the mixture before dividing it into the serving dishes, on top of the rhubarb.
Stand these in an appropriately sized roasting tray filled with water half way up the containers, and bake in the pre-heated oven for around 25 minutes until the custard mixture has set but is still slightly wobbly in the centre.