In a large bowl, place the flour, xanthan gum, cornstarch, baking powder, salt and 1/2 teaspoon garlic powder, and whisk to combine well. Add the grated cheese, shredded butter, and 1 tablespoon of chopped parsley, and toss to coat.

Create a well in the center of the dry ingredients and add the buttermilk. Mix gently, just until the dough begins to come together.

Line a rimmed baking sheet with unbleached parchment paper and set it aside.

Divide the dough into 10 equal pieces, roll each gently into a ball, flatten lightly into a dish about 3/4 of an inch thick and place about 2-inches apart from one another on the prepared baking sheet. Place the biscuits in the freezer to chill until firm (about 10 minutes).

Remove the baking sheet from the freezer. Preheat your oven to 375°F. Add the remaining 1/2 teaspoon garlic powder and remaining 1 tablespoon chopped parsley to the 2 tablespoons of melted butter, and mix to combine. Using a pastry brush, brush the tops and sides of the chilled biscuits with the garlic butter.

Place the baking sheet in the center of the preheated oven and bake until firm to the touch, brown on the underside and lightly golden brown on top (about 20 minutes). Remove from the oven, and allow to cool for 5 minutes before transferring to a wire rack to cool completely.