Combine 150ml of water and 150g of caster sugar in a saucepan and bring to the boil while whisking to dissolve

Remove from the heat and whisk in the chocolate until melted. Allow to cool slightly and add the Grand Marnier and remaining 600ml of water. Using a blender, blitz until smooth

Place the mixture into an ice cream maker and churn until a sorbet consistency is reached (If the sorbet mix is too thick to pour into the ice cream maker, whisk thoroughly to loosen. Freeze until ready to use