Line a 9-in. pie plate with pastry; trim and flute edges. Cut out scraps with a leaf-shaped cookie cutter for garnish if desired; place on a baking sheet and set aside.
In a large bowl, combine the sugar, cornstarch and cinnamon. Beat in the squash, butter, eggs, water and vanilla until smooth. Pour into crust.
Cover edges loosely with foil. Bake at 350° for 15 minutes. Remove foil. Bake 35-40 minutes longer or until a knife inserted in the center comes out clean. Bake leaf cutouts for 5-7 minutes or until golden brown.
Cool pie and cutouts on a wire rack for 1 hour. Refrigerate pie until chilled. Garnish with pastry leaves and whipped cream if desired.