Make the crust: Pulse the flour, granulated sugar and salt in a food processor to combine. Add the shortening and half of the butter, a few pieces at a time, and pulse until it looks like coarse meal. Add the remaining butter and pulse until the dough begins forming a ball. Add the vinegar and 2 tablespoons ice water; pulse a few more times until the dough is smooth. Turn the dough out onto a lightly floured surface and squeeze together, then roll out into a 12-inch round. Ease into a 9-inch pie plate, pressing the dough into the bottom and up the side. Fold the overhanging dough under itself and crimp with your fingers or a fork. Freeze until firm, about 1 hour.
Put a baking sheet on the middle oven rack and preheat to 375 degrees F. Line the crust with foil, then fill with pie weights or dried beans. Transfer to the hot baking sheet and bake until lightly golden around the edge, about 20 minutes. Transfer to a rack, remove the foil and weights and let cool completely; leave the baking sheet in the oven.
Meanwhile, make the filling: Whisk the pumpkin puree, melted butter, eggs, brown sugar, granulated sugar, allspice and cinnamon in a medium bowl until well combined. Add the buttermilk and flour and whisk until smooth. Pour into the cooled crust and return to the hot baking sheet in the oven. Bake until the filling is set, 50 minutes to 1 hour (the top may crack slightly). Transfer to a rack and let cool completely. Dust with confectioners' sugar just before serving.