In a small bowl combine white rice flour, tapioca starch, baking soda, xanthan gum, salt and sugar.
Use a whisk to combine the dry ingredients.
Add dry ingredients to the bowl of your stand mixer.
With machine on low add cubes of shortening and butter to dry ingredients, until mixture resembles a coarse meal.
Do not overmix.
With machine on low slowly add milk until dough comes together.
Remove from bowl and form dough into a ball.
Cover ball with plastic wrap and refrigerate for 1-2 hours.
Roll out dough onto a piece of parchment paper, lightly dusted with tapioca starch.
Dust top of dough with tapioca starch to prevent dough from sticking to rolling pin.
Cut out circle shapes to fit top of soup bowls.