Pears: Combine water and sugar in saucepan; bring to a boil. Simmer uncovered 5 minutes. Meanwhile pare, halve and core pears. Place halves cut side down in syrup and simmer till tender, about 5 minutes. Remove from heat, add vanilla and cool in syrup about 1 hour. (Pears may be cooked ahead of time and held in refrigerator till ready to serve.) Sauce: Melt chocolate and butter in double boiler (or in microwave). Gradually stir in boiling water, s