Preheat the oven to 375 degrees F. Line a baking sheet with parchment paper.
Place the starches, xanthan gum, baking powder, baking soda, salt and sugar into the bowl of a food processor. Process with six 1-second pulses.
Sprinkle the butter cubes evenly over the flour mixture, cover, and pulse for twelve 1-second pulses. The mixture should resemble coarse crumbs.
Pour the buttermilk evenly over the mixture. Process for eight 1-second pulses, or until the dough gathers into a moist clump.
Using your bare hands, quickly gather a clump of dough, lightly form it into a biscuit shape, and drop it onto the cookie sheet. Repeat this for all the remaining dough, trying to handle the dough as little as possible. You should get about 8 biscuits.
Bake 10-12 minutes or until lightly browned. Let cool for 20-30 minutes before eating.