Photograph: Gareth Morgans
Put the sugar and cocoa powder in a large pan with 500ml water. Bring to the boil, stirring, until the sugar and cocoa powder have dissolved. Remove from the heat and add the vanilla extract, chopped chocolate and glucose.
Stir until smooth and combined. Leave to cool a little, then blitz the mixture in a blender in batches. This isn't essential but will help give your finished sorbet a smooth texture. Pour into a bowl and leave to cool then chill for a couple of hours.
Churn the mixture in an ice-cream machine until frozen (make sure you churn it thoroughly), then transfer to a container. If you don't have an ice-cream machine, pour into a container, cover and freeze, stirring the mixture every hour until frozen.
When the sorbet is frozen, cover with a piece of clingfilm, pressing it down on to the surface of the sorbet to prevent ice crystals forming. Cover with a lid and freeze overnight. Serve in small scoops. Beware: it melts quite fast, so eat quickly!
Liquid glucose helps give this low-fat sorbet its silky texture – you'll find it in the baking aisle.