How to Make It
Prepare the Crust: Unwrap chilled pie dough disks from Double-Crust Pie Pastry, and place on a lightly floured surface. Let stand at room temperature until slightly softened, about 5 minutes. Sprinkle each disk with flour. Roll 1 disk into a 12-inch circle. Carefully fit dough circle into a 9-inch deep-dish glass pie plate, leaving a 1 1/2-inch overhang. Refrigerate until ready to use. Roll remaining disk into a 10-inch circle, and refrigerate until ready to use.
Prepare the Sweet Potato Filling: Place whole, unpeeled sweet potatoes in a large pot with water to cover by 2 inches. Bring to a rolling boil over high. Reduce heat to medium, maintaining a gentle boil. Cook until sweet potatoes are just tender enough to be sliced, but not so tender that they fall apart, 25 to 35 minutes. (Remove any smaller sweet potatoes as they are done, allowing larger ones to cook until they reach the ideal texture.)
Stir together sugar, flour, allspice, ginger, nutmeg, and cloves in a small bowl.
Drain sweet potatoes, and transfer to a platter to cool. Peel potatoes; trim and discard any fibers. Cut potatoes crosswise into 1/4-inch-thick rounds. (You will need about 4 cups to generously fill piecrust.) Gently toss sweet potatoes with sugar-spice mixture.
Preheat oven to 350°F. Cover bottom of piecrust in pie plate with a layer of Sweet Potato Filling; continue to layer to fill piecrust. Add additional filling to center, building it up a little higher than outer edges. Sprinkle all remaining sugar-spice mixture in bowl over top of pie; drizzle with sorghum syrup, and dot with butter pieces.
Carefully place 10-inch dough circle over filling. Fold edges of bottom crust up and over edges of top crust, and press firmly to seal. Using the tines of a fork, press dough around piecrust edge to make a decorative design. Using a sharp knife, cut 8 slits in top piecrust for steam to escape.
Prepare the Egg Wash: Stir together egg and water in a small bowl. Using a pastry brush, brush egg mixture evenly over piecrust. Sprinkle sugar over crust, if desired. Place pie on a baking sheet.
Bake in preheated oven until crust is browned, filling is bubbly, and sweet potatoes are tender all the way through, about 1 hour.
Transfer pie to a wire rack, and cool to room temperature, about 4 hours. Serve Molasses Whipped Cream with pie.