Slice the butter into small cubes, place them in a bowl, and place the bowl in the freezer to chill.
Preheat the oven to 425 °F
In a large mixing bowl combine the brown rice flour, sweet white rice flour, tapioca flour, cornstarch, potato starch, xanthan gum, baking powder, baking soda, sugar, salt and pepper. Whisk together until completely combined.
Cut the cold butter pieces into the flour mixture using a pastry blender, two knives, your fingertips, or a food processor fitted with the "S" blade, until the mixture resembles coarse crumbs.
Add the buttermilk to the bowl and gently mix until the dough begins to come together.
Lay a large piece of plastic wrap over your kitchen counter and turn the dough out on top of it. Working quickly, gather the dough into a mass and gently pat into a thickened of 3/4 -inch.
Cut the dough into 2 1/2 -inch squares with a knife or not 2 1/2 inch rounds with a floured biscuit cutter.
Arrange the biscuits no more than 1-inch apart on a baking sheet and bake in the oven for 12-15 minutes or until golden and cooked through.