How to Make It
In a food processor, pulse the flour, sugar and salt to blend. Add the butter and shortening and pulse until the mixture resembles coarse meal. Drizzle in 3 tablespoons of the ice water and pulse just until the dough can be gathered into a ball; if necessary, gradually add the remaining water. Turn the dough out onto a lightly floured surface and form it into a 9-inch log. Cut off a 4-inch section of the log and pat it into a 6-inch disk for the bottom crust. Wrap the disk in wax paper. Cut the remaining dough in half and wrap each piece separately in wax paper. Refrigerate all of the dough until chilled, at least 1 hour, or overnight.
On a lightly floured surface, roll the disk of dough into a 9-inch round and fit it into the base of a 9-by-2 1/2-inch springform pan. With your hands, roll each of the remaining dough pieces into a 13-inch rope. Fit the ropes around the base of the pan and join the ends to make one continuous rope. Using your fingers, evenly press the dough 2 inches up the sides of the pan to form a seamless shell. Refrigerate the shell.
Preheat the oven to 350°. Bake the sweet potatoes on a baking sheet for about 1 hour, or until soft when pierced. Let cool, then remove the skins and puree the potatoes in a food processor.
In a medium bowl, combine the granulated sugar and the brown sugar with the cinnamon, nutmeg and salt. In a large bowl, lightly whisk the eggs. Whisk in 3 cups of the pureed sweet potatoes (reserve any extra for another use). Whisk in the sugar mixture. Spoon half the sweet potato custard into the pie shell and sprinkle half of the chocolate on top. Cover with the remaining sweet potato custard and distribute the remaining chocolate decoratively on top.
Bake the pie in the center of the oven for about 1 hour, or until the custard is almost set in the middle. Turn off the oven but leave the pie inside until cool.
In a chilled bowl, combine the cream with the sugar, bourbon and cinnamon and beat until stiff. Serve the pie at room temperature and pass the whipped cream separately.