Preheat the oven to 350°.
Make a water bath by placing the butternut squash in a 13x9-inch pan. Fill the pan with water halfway up.
Bake the squash for 30 minutes or more, depending on the size of your squash. You should be able to put a fork through the squash.
Remove the squash from the pan and place it on a cutting board to easily peel off the skin.
Discard the pulp and the seeds. Remove the squash flesh with a spoon, measuring out 1-1/2 cups.
In a blender, mix the butternut squash, brown sugar, cornstarch, egg, evaporated milk, cinnamon, ginger, nutmeg, cloves, and the optional allspice if you are using it.
Puree it until it is smooth and there are no lumps.
Pour the filling into the unbaked pie shell. Cover the edges of the pie with a pie shield.
Bake at 350° for 45-50 min. or until a knife inserted halfway between the crust and the center comes out clean.
Cool the pie on a wire rack.
Store the squash pie covered in the refrigerator.