Stir together sugar, cocoa, and 3 cups water in a saucepan. Bring to a low boil over medium heat, whisking occasionally. Boil, whisking constantly, 1 minute. Remove from heat. Add chocolate morsels, and whisk until melted. Whisk together half-and-half and vanilla; whisk into chocolate mixture. Pour into a 9- x 5-inch loaf pan. Freeze 5 to 6 hours or until ice crystals form, stirring every hour. If desired, drizzle each serving with cold coffee or Irish cream liqueur.