For the pie crust
3 cups (420 g) all purpose gluten free flour (I highly recommend my Better Than Cup4Cup blend), plus more for sprinkling
1 1/2 teaspoons xanthan gum (omit if your blend already contains it)
1/2 teaspoon baking powder
3/4 teaspoon kosher salt
12 tablespoons (168 g) unsalted butter, grated and chilled
1 cup (240 g) sour cream, chilled
Ice water by the teaspoonful, as necessary
Egg white, for brushing
For the filling
22 ounces canned pumpkin puree
6 eggs (300 g, weighed out of shell), at room temperature
1 1/2 cups (12 fluid ounces) evaporated milk, at room temperature
1 cup (8 fluid ounces) heavy whipping cream, at room temperature
6 tablespoons (84 g) unsalted butter, melted and cooled briefly
1 1/2 cups (328 g) packed light brown sugar
1/2 cup (100 g) granulated sugar
3/4 teaspoon kosher salt
1 tablespoon pumpkin pie spice
1/4 cup (36 g) all purpose gluten free flour (any of my preferred blends will do, with or without xanthan gum)
Make the pie crust dough. In a large bowl, place the flour, xanthan gum, baking powder, and salt, and whisk to combine well. Add the grated, chilled butter, and toss to coat it in the dry ingredients. Add the sour cream, and mix to moisten the dry ingredients with the sour cream. The dough should be shaggy and somewhat crumbly. Knead the dough together with clean hands until it begins to come together. Add ice water by the teaspoon only if necessary for the dough to hold together. Press the dough into a rough disk, wrap it tightly in plastic wrap, and place it in the refrigerator to chill for 30 minutes.
Once the dough has chilled, turn it out onto a lightly floured surface. Sprinkle the dough lightly with more flour, and roll it out with a rolling pin into a rectangle that is about 1 inch thick, moving the dough frequently and sprinkling it lightly with flour if it begins to stick. Fold the dough over on itself like you would a business letter. Sprinkle the dough again lightly with flour, and roll out it out once again into a rectangle about 1 inch thick. Once or twice more, sprinkle the dough lightly with flour, roll it out, and fold it over on itself like you would a business letter, then wrap it tightly and place it back in the refrigerator to chill for about 2 hours. When the pie dough has nearly chilled completely, preheat your oven to 375° F and locate a 10-inch x 15-inch x 2-inch jelly roll pan. If your pan isn’t nonstick, coat it lightly in cooking oil spray and set it aside.
Prepare the filling. In a large bowl, place the pumpkin puree, eggs, milk, cream, and melted butter, and whisk vigorously to beat the eggs and combine the mixture very well. It should be very smooth. Add the brown sugar, granulated sugar, salt, pumpkin pie spice, and flour, whisking until just combined after each addition. The mixture should be about the consistency of half and half.
On a very lightly floured surface, place the chilled pie dough, slice off about 1/4 of the dough and set it aside. Roll out the remaining dough into an 18-inch x 12-inch rectangle. It should be about 1/4-inch thick. Trim any especially jagged edges. Fold the crust in half, and lift it carefully onto the prepared jelly roll pan in the proper orientation and into the edges of the pan. Fold any slightly overhung edges under themselves to secure them, and crimp the edges if you like. Brush the bottom of the crust lightly with the egg white, and place it in the refrigerator to stay cold. Preheat your oven to 375° F.
Working with the remaining, small piece of pie crust dough, roll it out about 1/4-inch thick, and cut out small shapes to place on top of the pie crust. You can either bake those shapes alone in a separate pan at 375°F for about 8 minutes or until lightly brown and set, or place them on the pie filling about halfway through baking.
Remove the pan with the bottom crust from the refrigerator and fill the crust about 2/3 of the way with the filling mixture. Pull out the oven rack, carefully place the pan on it, and quickly pour the remaining filling into the crust. Carefully return the oven rack to the oven, close the door, and bake for 20 minutes. If you’re adding any shapes to the top of the filling, open the oven and, working quickly, place the shapes on top of the filling. Reduce the heat of the oven to 350°F and bake until the edges are puffed and set, and the center jiggles only slightly and in a controlled, rather than a loose, way when the pie is shaken from side to side (about another 20 minutes). Remove the pie from the oven and gently press any separately baked pie crust shapes on top, or reserve them for serving on each piece. Place the pan on a cooling rack to cool for at least 45 minutes or until cool to the touch. Cover the pan tightly with plastic wrap and place it in the refrigerator to chill until firm (about 1 1/2 hours). Slice into squares and serve.