First, whisk the flours, salt, sugar, and xanthan gum together in a large metal bowl, then chill the flour in the fridge for 15 minutes. Then remove the bowl, take the butter out of the fridge (it should be very cold, but not frozen) and slice into 1/2 inch cubes. toss the butter cubes in the flour to coat. Carefully press each chunk so that it separates into two flatter pieces, and toss everything again to coat. Slowly pour in 1/2 cup of ice-cold water (I put a handful of ice cubes into a large glass of chilled water) and, using your hands, toss everything together again to combine. This is hydrating your dough! Continue to add about a tablespoon of ice water at a time, tossing everything and carefully pressing it together to feel if the dough is hydrated. The dough should be able to be pressed together without crumbling apart, but should NOT be wet or sticky. Do not add too much water, and do not mix the butter completely into the flour (the large bits of butter will make a flakier dough).
Press the dough together, then slice into two equal halves. Lightly flour a surface and carefully roll out each half and fold the rolled out dough twice, then press together again into a round disk (this will ensure that each chunk of butter is fully coated in flour - if it isn't, the butter will melt out of the crust). Wrap each disk in plastic wrap and refrigerate for 30 minutes.
Now, parbake the bottom crust! Preheat the oven to 425° then remove one of the crusts from the fridge. Lightly flour a waxed-paper lined surface, and place the unwrapped crusts on the surface. Lightly flour the top of the crust, and the rolling pin, and then roll out the crust to 1/4 inch thickness, at least 10 inches in diameter. Lift crust into the pan (or put the 9-inch pie tin upside down on the crust and lift and turn it over). Carefully press the crust into the pan, and use scissors to trim the extra crust around the edge so that an inch beyond the edge of the pan remains. Then tuck the edge of the crust underneath itself so that it forms a thick edge, then crimp the edge how you want to! Then, refrigerate the prepared crust, covered in plastic, for 30 minutes before baking (the butter+crust must be cold before putting it into the oven).
Once the prepared crust is chilled, remove from fridge, and use a fork to dock the crust (poke holes in it) all across the bottom and along the sides. Then place a large piece of parchment paper in the crust, and fill with pie weights (or if you don't have any, scoop a few cups of dry uncooked beans or white sugar into the parchment paper to weight it down), this will keep your crust from puffing up/drooping. Bake for 15 minutes at 425 to par-bake the crust. While it's baking, whisk together the one egge and teaspoon of water to create an egg wash. When done, remove the pie weights and immediately brush the entire inside of the crust with some of the egg wash. Let the crust cool completely before filling.