Place all the ingredients for the pannacotta into a medium saucepan and bring to the boil. Leave to boil for at least 2 minutes to activate the agar agar and leave to cool for 5 minutes before pouring in your moulds.
Add a couple of tbsp of the rhubarb juice into the pannacotta and give it a swirl.
Divide the pannacotta over 6 moulds. Place into the fridge at least four hours or preferably all night.