In a medium-size bowl, place the flour, xanthan gum, cornstarch, buttermilk or milk powder, baking powder, baking soda, salt, and granulated sugar, and whisk to combine well.

Store in an airtight container until ready to use. It’s best to store the mix in the refrigerator, especially if you’ve used buttermilk powder.

Preheat your oven to 400°F. Line a rimmed baking sheet with unbleached parchment paper, and set it aside.

In a large bowl, place the entire dry mix from the first step, and whisk to loosen.

Add the shredded and chilled butter, and toss to coat the butter in the dry ingredients, breaking up any large clumps in the butter. Add the mix-in berries or chips, and toss to coat.

Create a well in the center of the dry ingredients, add the cold water, and mix to combine. The mixture should come together and everything should be just moistened.

Turn the dough out onto a lightly floured surface, and pat into a square, handling it as little as possible. Working quickly, roll out with a rolling pin until the square is about 3/4-inch thick.

Press a metal bench scraper or other flat tool against the edges of the square to smooth any cracks in the dough and square the edges.

Using a large knife or bench scraper, cut the dough into 4 equal squares, then each square into 2 triangles by slicing it in half width-wise.

Place the triangles of dough about 1-inch apart from one another on the prepared baking sheet. Brush the tops with a bit of milk, and sprinkle with some coarse sugar, if you like.

Place the baking sheet in the freezer for about 15 minutes, or until the scones are firm.

Place the baking sheet with the chilled scones in the center of the preheated oven. Bake until the scones are puffed and firm to the touch, and just beginning to brown on the edges (about 20 minutes).

Remove from the oven and allow to set briefly before serving warm or at room temperature.