Preheat oven to 300°F. Line baking sheet with parchment paper. In small bowl, whisk together white rice flour, sweet rice flour, cornstarch, salt, baking powder, and xanthan gum. In bowl of stand mixer fitted with paddle attachment, cream butter, sugar, and vanilla extract together, on medium-low speed, until a thick paste forms, about 30 seconds. Shut off stand mixer. Add dry ingredients. Turn on mixer and mix until a dough forms, about three minutes.