Have eggs and butter cold for best results. Combine flours, starches, salt, xanthan gum, and sugar into a mixing bowl
Cut cold butter into slices and then work it into the flour mixture with hands or a pastry cutter until the dough feels slightly moist and begins to hold together
Add the beaten egg and vinegar to the flour mixture and stir with a spoon or fork until it begins to stiffen
The dough will be quite soft at first but will firm up. Is it firms up, form it into a ball and work it a little with your hands, use a little tapioca flour if necessary to keep your hands from getting sticky
Roll the dough out between two pieces of wax paper, turning and peeling off paper as necessary to smooth out wrinkles
Leave the paper on the pie dough to turn it. When it is ready for a pie pan, peel the top layer of paper off, hold the lightly greased pan over the dough, and slip your other hand under the bottom paper and dough
Lift it into the pan as you flip it all over
Smooth the dough into the pan before removing the wax paper
Again peel it off; don't lift it off. Crimp edges as desired. Prick with a fork if a baked pie shell is desired and bake at 350 degrees F for 12-15 minutes.
