In a medium saucepan, add 1½ cup of water, granulated sugar, cocoa ,and salt. Put the pot on medium to high heat and stir until boiling. Bring to the boil for about 1 minute, stirring constantly until the ingredients are melted and the mixture is homogeneous. Remove the pan from the heat, add the chopped chocolate and stir to melt the chocolate. Then add the remaining water, juice, and lemon zest and stir. Allow to cool well and freeze in an ice cream maker.