Heat 125 ml of the watermelon juice in a small saucepan together with the sugar and the salt. Heat until all sugar is dissolved.
Take of the heat and stir in the rest of the 675 ml watermelon in a medium bowl
MIx the lime juice together with the grand marnier in the watermxelon mix
Leave to cool thoroughly before putting it in the icemachine maker. Freeze in the machine according to the instructions of the manufacturer.
Ad the chocolate the last minute of churning
Notes
I used a watermelon that had virtually no pits. Clean the watermelon, remove any seeds where needed and puree the flesh in a blender. You can pass it through a sieve if you feel that is necessary. One large watermelon should give you more then enough juice