Combine the first 3 ingredients in a medium saucepan; bring to a boil. Cook 2 minutes or until sugar dissolves, stirring frequently.
Place the cocoa and chocolate in a medium bowl. Pour hot sugar mixture over chocolate mixture; let stand 2 minutes. Stir with a whisk until smooth. Stir in peppermint oil. Cover and chill.
Pour mixture into the freezer can of an ice-cream freezer; freeze according to manufacturer's instructions. Spoon sorbet into a freezer-safe container; cover and freeze 4 hours or until firm.