Combine water, sugar, and vanilla in a large saucepan and boil for several minutes. Then add the pears, and simmer, covered, for 10-15 minutes, or until tender.
(Sometimes a hard pear will require as much as 30-50 minutes cooking time.) Drain and cool. Melt chocolate. Add corn syrup. Remove from heat. Add vanilla and butter. Stir well. To serve: Arrange pears upright on serving dish. Place scoop of vanilla ice cream on plate. At the last minute pour the chocolate sauce over the pears. Remaining sauce may be passed.
