Put the sugar in a pan with 600ml (1pt) water. Bring to the boil, stirring until the sugar dissolves. Simmer for 5 minutes until syrupy.
Finely chop the chocolate and place in a bowl with the cocoa. Pour in a little hot syrup, stirring until the chocolate melts, then gradually stir in the remaining syrup until the mixture is smooth. Leave to go cold.
Churn the mixture in an ice-cream maker for 30 minutes until frozen. Alternatively, transfer to an airtight, freezer-proof container and freeze for 6 hours. Stir every hour to break up the ice crystals.
To serve, lightly crush the raspberries with the icing sugar. Spoon into cups ad place the sorbet on top.
Garnish with a sprig of mint.
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