Put the sugar, cocoa powder and the water into a saucepan; bring to the boil, stirring occasionally to prevent the sugar sticking. Simmer for 5 minutes.
Take off the heat,then add the roughly chopped chocolate. Stir until the chocolate has melted, then leave the mixture to cool.
Pour into an ice cream maker and churn until frozen OR place in a freezer-proof container and freeze. Whisk after 45 minutes with a fork to break up the ice crystals. Repeat 3 times then freeze for at least 6 hours.
Remove from the freezer 15 minutes before serving to soften.