For the panna cotta, preheat the oven to 140C/275F/Gas 1.
Place the orange rind onto a baking tray and bake in the oven for 20 minutes, or until completely dry.
Heat the dried rind, sugar, cream and vanilla together in a small pan and simmer until the sugar has dissolved. Remove the orange rind and vanilla.
Squeeze the excess water from the softened gelatine then add to the cream mixture and continue to simmer until dissolved. Remove from the heat and add the buttermilk.
Place four dariole moulds or ramekins onto a baking tray and pour in the panna cotta mixture. Chill in the fridge for 1-2 hours, or until set.
For the roasted rhubarb, preheat the oven to 200C/400F/Gas 6.
Place the rhubarb in a roasting tray, sprinkle over the sugar, then drizzle over the orange juice.
Roast the rhubarb in the oven for 8-12 minutes, or until just tender.
To serve, remove the panna cotta from the fridge. Briefly dip the base of each mould into a bowl of boiled water. Gently slide a knife around the edge of each panna cotta and turn out into the centre of a serving plate. Place the roasted rhubarb on the side and drizzle over any juice left in the pan.
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