Put the dried seaweed into a large container, fill with water and leave to soak overnight. Drain the seaweed.
Pour the sugar syrup in a saucepan and heat to 80°C (175°F). Add the rehydrated seaweed, remove from the heat and leave to cool.
Remove the seaweed and place in a bowl with the caster sugar. Dust the seaweed to coat completely, then transfer to a wire rack. Leave to dry