Preheat the oven to 180oC/Fan 160oC/Gas 4 and line a baking tin with baking parchment or foil. For the roasted plums, cut the fruits in half – don’t worry about removing the stones at this stage.
Arrange in the lined tin, cut-side uppermost, and sprinkle with the sugar. Split the vanilla pod in half down its length and cut each piece in half again.
Tuck the vanilla pieces and cinnamon stick amongst the plums and roast on the middle shelf of the oven for 30–40 minutes until the fruit is very tender, juicy and starting to caramelise at the edges.
Once the plums have roasted, leave them to cool, then scoop all the fruit and juice into a bowl, picking out and discarding the stones, vanilla pod pieces and cinnamon stick as you do so.
Whizz the plums until smooth – we find this easiest using a stick blender, but otherwise transfer to a food processor – and then pass through a fine-mesh nylon sieve if you want a silky smooth sorbet.
Pour 200ml cold water into a saucepan and add the caster sugar. Bring to the boil over a medium heat, stirring to dissolve the sugar.
Simmer for 2 minutes, then remove from the heat and add the roasted plum purée. Leave to cool, then cover and chill in the fridge for a couple of hours before churning in the ice-cream maker according to the manufacturer’s instructions.
An ice-cream maker will make a lighter sorbet, but if you don’t have one, simply freeze the mixture in a plastic freezer box, whisking it every couple of hours to break up the ice crystals.
Once the sorbet has frozen, break it into manageable chunks, tip into a food processor and blend until smooth and light. Return to the freezer box and freeze until firm.