For the Pie Dough:
1¼ cups unbleached all-purpose flour
½ teaspoon table salt
1 tablespoon granulated sugar
4 tablespoons unsalted butter, chilled, cut into 1/4-inch pieces
3 tablespoons vegetable shortening , chilled
4 - 5 tablespoons ice water
For the Sweet Potato Filling:
2 pounds sweet potatoes
2 tablespoons unsalted butter , softened
2 egg yolks
1 cup granulated sugar
½ teaspoon ground nutmeg
¼ teaspoon salt
2 tablespoons bourbon
1 tablespoon molasses
1 teaspoon vanilla extract
2/3 cup whole milk
¼ cup packed dark brown sugar
1. In food processor bowl fitted with steel blade, pulse flour, salt, and sugar to combine. Scatter butter pieces over flour mixture; cut butter into flour with five 1-second pulses. Add shortening and continue cutting in until flour is pale yellow and resembles coarse cornmeal, with butter bits no larger than small peas, about four more 1-second pulses. Turn mixture into medium bowl.
2. Sprinkle 4 tablespoons ice water over mixture. With rubber spatula, use folding motion to evenly distribute water into flour mixture until small portion of dough holds together when squeezed in palm of hand; add up to 1 tablespoon more ice water if necessary. Turn dough onto clean, dry work surface; gather and gently press together into cohesive ball, then flatten into rough 4-inch disk. Wrap in plastic and refrigerate at least 30 minutes, or up to 2 days, before rolling.
3. Remove dough from refrigerator (if refrigerated longer than 30 minutes, let stand at room temperature until malleable). Roll dough on lightly floured work surface or between two large sheets of plastic wrap to 12-inch disk about 1/8 inch thick. Fold dough in quarters, then place dough point in center of 9-inch pie plate; unfold dough.
4. Working around circumference of pan, ease dough carefully into pan corners by gently lifting dough edges with one hand while pressing around pan bottom with other hand. Trim edge to ½ inch beyond pan lip. Tuck rim of dough underneath itself so that folded edges are about ¼ inch beyond pan lip; flute dough. Refrigerate pie shell for 40 minutes, then freeze for 20 minutes. (This two-step process helps to reduce shrinkage of the crust during baking.)
5. Meanwhile, adjust oven rack to middle position and heat oven to 375 degrees. Press doubled 18-inch square of heavy-duty foil inside shell and fold back edges of foil to shield fluted edge; evenly distribute about 2 cups metal or ceramic pie weights over foil. Bake, leaving foil and weights in place until dough dries and lightens in color, 17 to 20 minutes. Carefully remove foil and weights by gathering sides of foil and pulling up and out. Bake until light golden brown, about 9 minutes longer. Remove from oven; reduce oven temperature to 350 degrees.
6. Prick sweet potatoes several times with fork and place on double layer of paper towels in microwave (see illustrations below). Cook at full power for 5 minutes; turn each potato over and continue to cook at full power until tender, but not mushy, about 5 minutes longer. Cool 10 minutes. Halve each potato crosswise; insert small spoon between skin and flesh, and scoop flesh into medium bowl; discard skin. (If potatoes are too hot to handle comfortably, fold double layer of paper towels into quarters and use to hold potato half). Repeat with remaining sweet potatoes; you should have about 2 cups. While potatoes are still hot, add butter and mash with fork or wooden spoon; small lumps of potato should remain.
7. Whisk together eggs, yolks, sugar, nutmeg, and salt in medium bowl; stir in bourbon, molasses, and vanilla, then whisk in milk. Gradually add egg mixture to sweet potatoes, whisking gently to combine.
8. Heat partially baked pie shell in oven until warm, about 5 minutes. Sprinkle bottom of pie shell evenly with brown sugar. Pour sweet potato mixture into pie shell over brown sugar layer. Bake until filling is set around edges but center jiggles slightly when shaken, about 45 minutes. (Check about halfway through - if the crust is browning too much, cover with foil.) Transfer pie to wire rack; cool to room temperature, about 2 hours, and serve.