Cut the rhubarb into 2cm chunks and put into a pan with the sugar and 500ml water. Split the vanilla pod, if using, scrape out the seeds and set these aside. Add the pod to the pan with the rhubarb. Bring to the boil, then lower the heat and simmer for 10 minutes. Leave to cool for half an hour or so.
Line a sieve with muslin and set this over a bowl. (Or use a jelly bag.) Pour the rhubarb and its juice into the sieve (or bag) and leave it to drip. Don’t force the juice through or you will make it cloudy. You want to end up with 350ml of pale, pink, clear juice. (Any excess can be added to the rhubarb pulp, chilled and stirred into yoghurt and/or muesli for breakfast.) If you don’t have quite enough juice, you’ll have to add a touch more Champagne to make up the difference. Shame. Put your 350ml juice in a small pan.
Soak the gelatine in a shallow bowl of cold water to soften for 5 minutes. Bring the rhubarb juice almost to a simmer and take off the heat. Squeeze the soaked gelatine leaves to remove excess water, then add to the rhubarb juice and stir until fully melted. Pour into a large jug and leave to cool to room temperature.
Open your chilled Champagne and very, very slowly pour 500ml into the syrup – I tilt the jug and trickle the Champagne down the side. This painstaking step will maximise the number of bubbles held within your jelly. Stir the mixture as carefully as you can to make sure the rhubarb liquid and wine are completely combined. You will find a frothy head forms on top of the liquid – just skim it off with a spoon. Carefully pour the fizzy jelly into wine glasses, skim again if necessary and place in the fridge to set. This should take 2–3 hours.
When you are ready to serve, add the reserved vanilla seeds, if using, to the cream, and whip until it holds soft peaks. Put a little spoonful of this on top of the jellies and serve.
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