Mix together flour, xanthan gum, salt, sugars in food processor or mixer.
Add butter pieces and mix until incorporated. I usually pulse or mix about 10 seconds.
Mix egg wth vanilla.
Stream in egg mixture.
Add cold water 1 teaspoon at a time until dough forms.
Turn dough onto sheets of plastic.
Flatten, wrap well in plastic and refrigerate for one hour.
Once you are ready to roll, take out and, if necessary, let it sit out on counter to soften a little.
Roll into a 12 inch round. Don't worry if it's not perfectly round.
Put dough in tart pan and reserve scraps to press in.
Place prepared tart pan in freezer for for about 15 minutes, prior to baking.
Bake and fill as needed for recipes.
If you need to bake completely without filling (for a fruit tart for example), bake for 25 minutes at 375 degrees F.
Cover with Parchment paper that has been greased or sprayed with a baking spray.
Fill with pie weights (or dried beans). Make sure to cover edges so they don't burn.
Cool completely before filling.