Tip the sugar into a bowl, sift in the cocoa and stir. Bring 600ml water to the boil in a medium saucepan. Whisk in the sugar and cocoa and return to a gentle simmer. Simmer, uncovered, for 5 mins, whisking occasionally. Remove from the heat and stir in the chocolate and vanilla until the chocolate has melted.
Cool the mixture, then put in the fridge for several hrs or overnight until well chilled. Make the sorbet in an ice-cream machine or pour into a rigid plastic container, preferably bowl shaped, and freeze until frozen 3cm from the edges. Whisk to break down the ice crystals, then freeze again for 1 hr more. Whisk and freeze again one more time, then leave until completely frozen.
Before serving, transfer to the fridge for 30 mins to make scooping easier.