This can be done in advance and freeze it. It will stay good in the freezer for up to 6 months.
· ¾ teaspoon baking soda
· ¾ teaspoon salt
· two sticks frozen unsalted butter, grated
· ½ cup cold water
· 1 egg, for an egg wash (or heavy cream)
Mix the white rice flour, tapioca starch, potato starch, xanthan gum, baking soda, and salt in a mixing bowl.
Grate cold butter into the flour mixture, stirring and coating the butter pieces as you go.
Once the butter pieces are the size of peas, make a well in the center and add the cold water. Use your hands to work the dough together and into a ball.
Put it in the fridge for up to 2 hours.
Remove the dough from the fridge and let it sit for 25 minutes. Pre-heat your oven to 400°F
Place the dough on a piece parchment paper and press the dough into a rectangle. If the dough cracks around the edges the dough is still too cold, let it sit at room temperature for another 25 minutes. If the dough is too soft refrigerate it for 20 minutes.