Lightly grease a baking sheet, or line it with parchment., Whisk together the flour, salt, xanthan gum, baking powder, and baking soda., Cut in the butter until it's in chunks about the size of a nickel., Stir together the buttermilk and honey and add, all at once, to the dry ingredients. Stir until the dough comes together with only a few dry crumbs remaining. Turn the dough out onto a piece of parchment or waxed paper that's been dusted with a little cornstarch., Use the paper to fold the dough over on itself eight times, patting it to a thickness of 1/2" between folds., After the last fold, pat the dough 1/2" to 3/4" thick, and cut with a 2 1/2" biscuit cutter. Stack the scraps on top of each other and pat down again, repeating the cutting process until all of the dough is used., Place the cut biscuits on the prepared baking sheet, and place them in the freezer. You can wrap any extra biscuits once they're frozen, and bake them as you need them later on., Preheat the oven to 425°F with a rack in the middle to upper third., When the oven is hot, remove as many gluten-free biscuits as you plan to bake, brush the tops with some milk or buttermilk, and bake for 15 minutes, until the tops are golden brown. Remove from the oven and serve warm.