Preheat oven to 375°F. Place 2 tablespoons butter in 8 x 8 x 2-inch glass baking dish. Place dish in oven until butter melts, about 3 minutes. Remove from oven. Arrange pears in dish.
Combine water and prunes in heavy small saucepan; bring to boil. Reduce heat; simmer until prunes are plump and liquid is reduced to 1/4 cup, about 8 minutes. Arrange prunes over pears. Drizzle with prune liquid. Sprinkle with brown sugar and cinnamon. Dot with 1 tablespoon butter. Bake until pears are tender, occasionally turning and basting with juices, about 35 minutes. Serve warm.
