Preheat oven to 400°. Line a baking sheet with parchment paper.
In a bowl, whisk together sorghum flour, sweet rice flour, potato starch, tapioca starch, sugar, baking powder, baking soda, xanthan gum, buttermilk powder and salt. With a pastry cutter or your hands, cut butter into flour mixture until it is crumbly and resembles a coarse meal. Add cheese, green onion and black pepper.
Beat egg whites in a separate large mixing bowl until very foamy. Add cold water and flour mixture to egg whites all at once. Mix with a rubber scraper or spoon until dough starts to come together.
Transfer dough onto a surface that has been lightly dusted with sorghum flour. If sticky, add 1–2 Tbsps sorghum flour to top of dough and pat into a 6x6-inch and 1-inch-thick square. Cut biscuits into nine even squares. Place about 1 inch apart on baking sheet, and brush lightly with the melted butter.
Bake 13–15 minutes. Let biscuits cool on baking sheet, and then serve immediately.