Whisk egg and vanilla together, set aside.
In a food processor add flour blend, brown sugar, icing sugar, xanthan gum and salt. Pulse a few times to combine.
Scatter butter pieces over top and pulse the mixture until there are no butter pieces larger than a pea. Don’t over mix.
With the processor running and egg mixture and mix until it just starts to form a dough.
Add cold water, 1 teaspoon at a time, pulsing a few times until you have dough.
Turn dough out onto wax paper or plastic wrap and push it into a large disk or two disks if making individual tarts. Wrap well with plastic wrap and refrigerate for 1 hour. (Can be made 1-2 days in advance up to this point.)
Take cold dough out of the fridge. Lightly spray or brush tart pan(s) with oil.
Top dough with a second piece of wax paper or plastic wrap and roll into a large circle 1 or 2 inches larger than your baking pan.
If the dough is sticky put it back in the fridge until cold or use a small amount of sweet rice flour to continue rolling. Use as little sprinkling flour as possible since too much can make the tender dough tough.
Remove top sheet of wrap and place tart pan upside on pastry. Push it into the dough then flip the whole thing over with the aid of a large cookie sheet or cutting board. The dough will be in the bottom of the pan. Remove the wrap.
Lightly press scraps of dough into the edges of the pan to make the sides. Using a flat bottom glass or measuring cup, gently run it over the entire bottom and up against the sides of the pan to ensure the dough is as even as possible.
Refrigerate for 15 minutes before baking.