For the roasted rhubarb, preheat the oven to 200C/400F/Gas 6.
Place the rhubarb onto a roasting tray, sprinkle over the sugar then drizzle over the apple juice.
Roast in the oven for 8-12 minutes, or until just tender.
Blend 125g/5oz of the roasted rhubarb to a purée in a food processor. Reserve the remaining roasted rhubarb.
For the panna cotta, bring the sugar and cream to a simmer in a pan, stirring continuously until all of the sugar has dissolved.
Add the softened gelatine leaves and continue to simmer until dissolved.
Add the rhubarb purée and stir until well combined, then remove from the heat and set aside to cool slightly.
Add the buttermilk and stir until well combined.
Place four dariole moulds or ramekins onto a baking tray. Place a piece of the reserved roasted rhubarb into the bottom of each of the dariole moulds and pour the panna cotta mixture on top. Chill in the fridge for 1-2 hours, or until set.
To serve, remove the panna cotta from the fridge. Briefly dip the base of each into a bowl of boiled water. Gently slide a knife around the edge of each of the four panna cotta and turn each out into the centre of a serving plate. Spoon the remaining reserved rhubarb, and the juices, around the edge of each of the four serving plates. (Reheat the roasted rhubarb if desired.)