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.

Lightly white rice flour your countertop. Turn dough out onto counter and shape dough into a disk. Using a rolling pin, roll dough out, about 1/4-inch thick. Cut using a round cutter and place cookies on prepared baking sheet, about two inches apart. Chill cookies for ten minutes.

Bake until cookies are lightly golden brown on the edges and aromatic, about 30 minutes. Allow cookies to cool on the pan for five minutes and then transfer to wire rack to cool. Repeat using remaining dough.

Store in an airtight container for up to two weeks.