Preheat oven to 450°F (232°C).
In a large bowl, whisk together the gluten-free flour, xanthan gum, baking powder, salt, and sugar until well combined.
Cut the butter into small pieces and place in the freezer for 10 minutes to chill.
Cut the chilled butter into the flour mixture with a pastry cutter or fork until the mixture resembles small peas.
Add the buttermilk and whisked egg to the flour mixture and stir until a soft, sticky dough forms, being careful not to overmix.
Lightly flour a large piece of parchment paper with 1 tablespoon of gluten-free flour.
Place the dough on the floured parchment paper, dust the top with 1 tablespoon of flour, and gently fold the dough over itself twice.
Form the dough into a round approximately 7 inches in diameter and 1 inch thick.