In a large bowl, whisk together the flour blend, xanthan gum, cornstarch, baking powder, baking soda, salt, and sugar.

Cut the chilled butter into ¾-inch cubes. If it softens, chill it again until firm.

Add the butter to the dry ingredients and toss to coat. Use your fingers to flatten each piece into thin shards.

Pour in ¾ cup of cold buttermilk and mix to form a dough. If it feels too dry, add more buttermilk, one tablespoon at a time, until it comes together (likely around 2 additional tablespoons).

Knead the dough into a rough disk and chill for about 5-10 minutes if it softens.

Lightly dust a flat surface with flour. Roll the dough into a rectangle about ¾-inch thick.

Fold it into thirds like a business letter, and roll it out again. Repeat this fold-and-roll process 1-2 more times, chilling as needed if the butter starts to soften.

Finally, roll out the dough to just under 1 inch thick. Use a lightly floured biscuit cutter to cut out 2 ½-inch rounds.

Place the rounds on a prepared baking sheet, spaced about 2 inches apart. Reroll scraps as needed.

Brush the tops with melted butter and sprinkle with coarse salt, if desired.

Chill the biscuits again until firm. Preheat your oven to 425°F (220°C).

Bake for about 15 minutes until puffed and pale golden. Allow them to cool briefly before serving.