Make the crust: Pulse the flour, granulated sugar and salt in a food processor to combine. Add the shortening and pulse until the mixture looks like cornmeal. Add the butter and pulse until it is in pea-size pieces. Add the vinegar and 2 tablespoons ice water; pulse until the dough just comes together but is still crumbly. Turn out onto a piece of plastic wrap; shape into a disk and wrap. Refrigerate until firm, at least 1 hour or overnight.
Roll out the dough into a 12-inch round on a lightly floured surface. Ease the dough into a 9-inch pie plate. Fold the overhanging dough under itself and crimp with your fingers or a fork. Refrigerate until firm, about 30 minutes.
Preheat the oven to 350 degrees F. Line the crust with foil, then fill with pie weights or dried beans. Bake until lightly golden around the edge, about 20 minutes. Remove the foil and weights and continue baking until golden all over, about 15 more minutes. Transfer to a rack and let cool completely.
Meanwhile, make the filling: Prick the sweet potatoes all over with a fork. Microwave until tender, about 10 minutes. Let cool slightly, then peel the sweet potatoes and transfer to a mini food processor; puree until very smooth, about 1 minute. (You should have about 1 cup sweet potato puree.)
Whisk the granulated sugar, brown sugar, cornmeal and salt in a medium bowl. Whisk the sweet potato puree, eggs, melted butter, buttermilk, lemon zest and juice and the vanilla in a large bowl; add the sugar mixture and whisk. Pour the filling into the cooled crust. Bake until the filling is set in the middle, 45 to 50 minutes. Transfer to a rack and let cool completely.
Photograph by Con Poulos
Courtesy of Food Network Magazine