10 g gelatin leaves (individual leaves vary in weight so check packet for info)
for the rhubarb
900 g (2lb) rhubarb
100 g (3½oz) caster sugar
grated zest and juice of 1 orange (200ml/7fl oz juice)
Place the cream, sugar, vanilla pods and seeds in a saucepan over a medium–high heat and bring to a gentle simmer. Stir until the sugar has dissolved, then remove from the heat. Meanwhile, place the gelatin sheets in a Pyrex or glass dish and cover with a little water. Leave to stand for 5 minutes until the gelatin has softened.
Remove the softened gelatin from the liquid and squeeze out any excess. Remove the vanilla pods from the warmed cream, then add the softened gelatin and stir through until dissolved. Divide the mixture amongst six pudding moulds, cover with cling film and place in the fridge to set for a few hours.
Cut the rhubarb into 6cm (2½in) pieces. Place the sugar, orange zest and juice in a cooking pot with 100ml (3½fl oz) water, place over a medium–high heat and bring to a steady simmer, stirring until the sugar is dissolved. Add the rhubarb to the pot and poach until it is soft when pierced with a fork. Remove with a slotted spoon and set aside on a plate.
Continue to cook the poaching liquid until it becomes syrupy. Remove from the heat and allow to cool slightly before adding the rhubarb back in.
To remove the panna cotta from the moulds, fill a bowl with warm water and dip them in. The heat will loosen them slightly and allow them to be inverted onto serving plates. Serve each panna cotta with a few tablespoons of the rhubarb mix.