Put the gelatin leaves in a bowl of cold water and put them in one by one. Leave to soak for five minutes.
In the meantime heat the double cream, sugar and lemon juice slowly until almost boiling in a heavy saucepan. Stir until the sugar is dissolved, then remove from the heat.
Squeeze the gelatin leaves until most fluid has gone out and stir them through the hot cream mixture til dissolved. Stir in the buttermilk and the vanilla marrow til everything is combined.
Pour the mixture into glasses, ramekins or custard moulds. You can use anything for that, depending on how you want to present it.
Cover with some plastic and leave to set for at least four hours in the fridge. Preferably a whole night if you have the time. At least chil it while you're making the rhubarb layer.
Clean the rhubarb and cut into chunks of about 2 cm. Remove the top and bottom of the stalks and throw those out.
Place them in a saucepan with a little bit of water, a cinnamon stick and some sugar. Don't add to much sugar but depending on the acidity of the rhubarb you can ad more or less. Taste as soon as the rhubarb starts to soften.
Remove the cinnamonstick once the rhubarb is done and pass through a sieve so you're left with just the fluids.
Soak 1 gelatin leave in cold water while you heat the rhubarb a tiny bit so it can be dissolved. No need to bring to the boil.
Leave to cool while you carefully pour the rhubarb on top of the pannacotta. Make a nice layer and put back in the fridge to set