In a large bowl, whisk the flour, sugar, and salt together. Add the cubes of butter and use a pastry blender (or two forks) to cut the butter in until it's incorporated and the mixture is coarse, with pea sized pieces of butter throughout.
1 ¼ cups (185g) gluten free flour blend, 1 teaspoon granulated sugar, ½ teaspoon fine sea salt, ½ cup (113g) unsalted butter
Add the ice water, one tablespoon at a time, and mix until dough just comes together when pressed. Use your hands to form into a round disc. Wrap in plastic wrap and chill in the refrigerator for 2 hours.
4-6 tablespoons ice cold water
Preheat the oven to 425°F. Remove the pie dough from the refrigerator. On a lightly floured surface, roll the dough out to a 12-inch circle. Transfer to a glass pie plate and crimp or trim the edges.
To make the filling, use a hand mixer or a stand mixer fitted with the paddle attachment to beat the pumpkin, condensed milk, arrowroot, pumpkin pie spice, and vanilla until well combined. Transfer to the pie crust.
Bake for 15 minutes. Then, without removing the pie from the oven, reduce the temperature to 350°F and continue to bake for 35-40 minutes, or until a knife inserted into the center of the pie comes out clean.
Notes
If the pie dough is too crumbly, add more water, one tablespoon at a time.