Step 1In a medium bowl, combine the flour, sugar and salt. Cover and freeze for 10 minutes.
Step 2In the bowl of a food processor, put the flour mixture, butter and shortening. Pulse to cut the fat into small pieces, some the size of peas and some smaller.
Step 3With the machine running, slowly pour the water through the feed tube. The dough should begin to come together into a ball; when it does, turn off the machine immediately. Do not let it form a complete ball.
Step 4Turn the dough out onto a floured work surface and gently shape it into two disks about 1 1/2 inches thick. Wrap the disks in plastic wrap and refrigerate for 30 minutes to 1 hour.
Step 1Remove one-half of the pastry dough from the refrigerator 10 minutes before rolling. Line a 9-inch deep-dish pie pan with the dough, draping it over the edge of the pie pan with a 1 1/2 -inch border of pastry extending beyond the rim. Refrigerate until ready to fill.
Step 2Place the whole, unpeeled sweet potatoes in a large pot with water to cover by 2 inches. Bring to a rolling boil over high heat. Reduce the heat to maintain a gentle boil, and cook until the sweet potatoes are tender enough to be sliced, but not so tender that they fall apart. Depending on their size and shape, this should take between 15 and 30 minutes. Remove any smaller sweet potatoes as they reach the right texture while the larger ones finish cooking.
Step 3While the sweet potatoes are cooking, prepare the seasonings. In a small bowl, combine the sugar, flour, allspice, ginger, nutmeg and cloves. Stir with a fork to mix them together well.
Step 4In a medium bowl or heatproof measuring cup, combine the cream and molasses. When the sweet potatoes are cooked, measure out one-half cup of their cooking liquid. Add this to the molasses and cream and stir to mix these liquids well.
Step 5Drain the sweet potatoes and set them out on a platter to cool enough to be handled. Peel and trim them. Slice them lengthwise (like planks, not crosswise) into slabs about one-fourth-inch thick. You'll need about 4 cups; enough slices to generously fill the piecrust.
Step 6Heat the oven to 350 degrees. Roll out the top crust to about 10 inches in diameter.
Step 7Have the spice mixture, molasses mixture and butter all ready. Place 2 layers of sweet potato slices in the bottom of the piecrust. Sprinkle about one-third of the spice mixture over the layered sweet potatoes. Add another 2 layers of sweet potato slices, another third of the spice mixture, and finish up with a final two layers of sweet potato slices, filling the piecrust almost to the very top. Add a few slices of sweet potatoes to the center, to build it up a little higher. Sprinkle all the remaining spices over this layer.
Step 8Pour the molasses mixture evenly over the filling, enough to moisten the sweet potatoes and the spice mixture (you may have extra molasses mixture. Just use what you need — we used almost half), and place the bits of cold butter around the top of the pie. Cover the pie with the top crust. Fold the edges of the bottom crust up and over the top crust and press to seal them together well. Using the tines of a fork, work your way around the piecrust, pressing to make a handsome parallel design on the crust as you seal it. Use a sharp knife to cut 8 slits in the top of the pie, so that steam can escape and the filling can bubble up through the crust.
Step 9Place the pie on a baking sheet in the center rack of the oven. Bake until the crust is nicely brown, the filling is bubbling, and the sweet potatoes are tender all the way through, 45 to 55 minutes.
Step 10Place the pie on a cooling rack or a folded kitchen towel and cool to room temperature.