Prepare the Crusts

1. Prepare the dough. In a large mixing bowl, combine the flour, kosher salt, and sugar. Cut in the vegetable shortening with a fork until the mixture resembles coarse, sandy crumbs.
2. Incorporate the butter. Add the cold, cubed butter and work it into the flour mixture with a pastry cutter. Continue until the mixture begins forming larger, irregular clumps of flour and butter, with visible pieces of butter remaining. Stop cutting once these clumps begin to form.
3. Add the water. Beginning with 2 tablespoons, gradually add ice water, mixing gently until the dough holds together when pressed. Avoid overworking the dough or adding more water than necessary.
4. Chill the dough. Gather into a ball, flatten into a disc, wrap tightly, and refrigerate for at least 30 minutes. Repeat steps 1–4 twice more to make three separate pastry discs.
5. Roll and shape. Working with one chilled disc at a time, roll the dough on a lightly floured surface to approximately ⅛–3/16 inch thick and about 12 inches in diameter. Transfer to a 9-inch pie plate without stretching. Trim to leave ½–1 inch of overhang, fold the excess underneath, and crimp the edge. Refrigerate the prepared shells until thoroughly chilled.
6. Prepare for blind baking. Preheat the oven to 425°F. Line each chilled crust with parchment paper and fill with pie weights to support the bottom and sides. Protect the crimped edges with a silicone pie crust protector or strips of aluminum foil.
7. Blind-bake the crusts. Bake each crust for 21 minutes. Carefully remove the pie weights, parchment, and edge protectors. Set aside until all three crusts have completed this initial bake, then proceed with the egg wash.
8. Seal the crusts. Beat one whole egg thoroughly and brush a thin layer over the bottoms and sides of all three crusts, avoiding pooling. Return to the oven for 4 minutes to set the egg wash. Remove and set aside while preparing the custard.

Prepare the Custard

1. Reduce the pumpkin. Place the pumpkin in a heavy-bottomed Dutch oven over medium-low heat. Cook, stirring frequently and scraping the bottom, until its weight decreases from 822 g to approximately 740 g. Avoid scorching or browning.
2. Incorporate the sugars and spices. Combine the brown sugar, granulated sugar, cinnamon, ginger, nutmeg, allspice, and salt. Stir into the reduced pumpkin and cook gently for 2–3 minutes, stirring constantly, until evenly incorporated and fragrant.
3. Prepare the dairy. In a separate saucepan or stockpot, whisk the cornstarch into ½ cup cold whole milk until smooth. Add the evaporated milk, heavy cream, and another 1½ cups whole milk. Heat gently, stirring, until the mixture is hot and the starch begins to thicken, approximately 185–190°F. Do not boil vigorously.
4. Combine and cool. Gradually stir the hot dairy mixture into the pumpkin mixture until smooth. Remove from the heat and allow to cool to approximately 110°F.
5. Add the eggs. Whisk the eight whole eggs and two additional yolks until uniform. Gradually whisk in some of the warm pumpkin mixture to temper the eggs, then incorporate them into the remaining custard. Stir in the vanilla.
6. Blend and strain. Briefly process the custard with an immersion blender, keeping the blade submerged to avoid incorporating excessive air. Pass through a medium-fine sieve, gently pressing the pumpkin solids through.
7. Adjust the consistency and yield. Measure the finished custard. Add whole milk in small increments as needed, aiming for about 12 cups while keeping the custard smooth and pourable. Do not dilute a properly balanced custard solely to reach the target volume.

Fill and Bake

1. Fill the shells. Reduce the oven temperature to 325°F. Place the prepared crusts on baking sheets and divide the custard evenly among them, leaving a small margin beneath the crimped rims.
2. Bake gently. Bake until the outer custard is set and the centers retain a gentle wobble. Begin checking after approximately 35 minutes. Use an instant-read thermometer to check doneness, aiming for a center temperature of about 170–175°F. Avoid letting the filling puff up too much, boil, or crack.
3. Cool and chill. Transfer the pies to wire racks. Allow to cool, then refrigerate until thoroughly chilled, preferably overnight.