Cut the butter into small pieces and freeze for 10 minutes.
Preheat the oven to 450°F. In a large bowl, add 2 cups flour, xanthan gum, baking powder, salt, and sugar, and stir to combine.
Cut butter into flour with a pastry cutter or fork until the butter is the size of small peas. Add buttermilk and whisked egg and stir until soft dough forms. Stir in scallions. Do not over mix or you will end up with tough biscuits. This dough will be sticky.
Add 1 tablespoon flour to a large piece of parchment paper. Place dough on top of floured parchment. Do not roll dough out. Dust top of the dough with 1 tablespoon of flour and gently fold dough over on itself 2 times by folding the dough in half on top of itself and then doing it a second time.
Form a round disc that is about 6-7 inches in diameter and 1 inch thick; any larger or flatter and you will end up with hard, flat biscuits. Cut out 2-inch biscuits using a biscuit cutter, the mouth of a glass, or lid of a Mason jar. Do not twist cutter when cutting, or biscuits will not rise well. Re-form dough scraps into another dough round and cut out more biscuits. Place biscuits on a greased baking sheet or in a greased 12-inch cast-iron skillet.
Bake biscuits for 15-20 minutes. Check at 15 minutes to see if they are golden brown. If not, continue to bake for another few minutes, checking regularly. If desired, once biscuits are out of the oven, brush with some melted butter and sprinkle with sea salt. Serve warm.