Peel and dice sweet potatoes. Boil until tender, about 20-25 minutes. Drain and mash with a potato masher. Put aside.
For the crust: In a big bowl combine flour, sugar, 1 ts allspice and the salt. Cut in butter (using a knife or your fingers) until mixture resembles coarse sand. Sprinkle 1 tbs of the ice water over the flour mixture and gently toss with a fork. Push moistened dough to side of bowl. Repeat moistening flour mixture, using 1 tbs of ice water at a time, until all mixture is moistened (you might need more than 5 tbs).
Form dough into a ball. On a lightly floured surface use your hands to slightly flatten the dough. Roll out into a circle of 12 inches in diameter.
Transfer dough to a 9 inch pie plate. If you have any extra dough, fold it under. Crimp edges as desired. Do not prick the dough.
Preheat oven to 190°C/ 375°F.
For the filling: From your sweet potato mash measure 1 1/2 cups. In a medium bowl combine this, brown sugar, cream, eggs, 1 1/2 ts allspice, vanilla, orange peel (if using) and salt.
Pour filling into prepared pie plate.
Bake for 40-45 minutes or until knife inserted near the centre comes out clean. NOTE that you might have to cover the pie after 25 minutes or so to prevent overbrowning.