Combine flour, xanthan gum and salt in medium bowl, whisk to combine well. 

Stir in butter chunks, mashing them into the dry ingredients. 

In a small bowl, whisk together the water and egg yolk.

Add water/yolk to the center of the dry ingredients, adding a teaspoon of cold water if necessary.

Knead lightly.

Refrigerate from 2 to 24 hours....

Preheat oven to 375°F.

Press into the bottom (up the sides is optional) of a 10-inch pie pan. 

Bake 15-25 minutes to desired brownness.

Can be used immediately for quiche, pot pie or tart.