Reserve 1/2 cup of the 8 ounces chopped chocolate; cover and set aside. In a medium saucepan, stir together remaining chopped chocolate, the water, sugar, and whole milk. Bring to boiling, whisking constantly. Boil gently for 1 minute. Remove from heat; stir in vanilla. Cover and chill overnight.
Stir the reserved 1/2 cup chopped chocolate into the chilled chocolate mixture. Freeze mixture in a 1-quart ice cream freezer according to manufacturers directions. Allow to firm up in freezer for 4 hours before serving.
To serve, scoop into small glasses or dishes. If desired, chop the 1 ounce chocolate; sprinkle over individual servings.