Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper.
Whisk together the flour, xanthan gum, sugar, baking powder, baking soda, and salt.
Add the grated frozen butter and toss until coated with the flour mixture.
Pour in the cold buttermilk and mix until a rough dough forms.
Gently knead just until the dough comes together.
Roll the dough to ¾–1 inch thick, cut it into four pieces, stack them, press gently, and roll again to the same thickness.
Cut out biscuits using a 2½-inch round cutter.
Place the biscuits on the prepared baking sheet and freeze for 15–20 minutes.
Arrange the biscuits so they’re lightly touching and brush the tops with melted butter.
Bake for 20–24 minutes, until golden brown and well risen.
Brush the hot biscuits with more melted butter and serve warm.
