To make the compote, place the rhubarb, sugar and the lemon juice in a pot and bring to the boil. Simmer on a medium heat for around 7 min. Mix the cornflour with a little cold water and add to the rhubarb mixture. Remove from the heat and leave to cool.
To make the panna cotta, soften the gelatine in cold water. Place the vanilla pod, sugar and the cream into a pot and bring to the boil. Wring out the gelatine and dissolve in the warm cream mixture (do not allow to boil any more).
Remove the vanilla pod and beat the mixture over a cold bain marie. When the mixture begins to gel, fold in the yogurt and transfer the mixture to small bowls.
Chilli for around 4 hours in the fridge and top with the compote to serve. Garnish with mint leaves.