3 cups (420 g) all purpose gluten free flour (I used Better Batter)
1 1/2 teaspoons xanthan gum (omit if your blend already contains it)
2 teaspoons baking powder
1 teaspoon baking soda
1/2 teaspoon kosher salt
1/4 teaspoon cream of tartar
2 tablespoons (25 g) granulated sugar (optional)
3 tablespoons (42 g) unsalted butter, grated and chilled
2 1/2 cups (20 fluid ounces) buttermilk, chilled
Preheat your oven to 375°F. Line a 10-inch cast iron skillet with parchment paper and set it aside.
In a large bowl, place the flour, xanthan gum, baking powder, baking soda, salt, cream of tartar, and optional sugar, and whisk to combine well. Add the grated and chilled butter, and toss to coat the butter in flour and evenly distribute it throughout the dry ingredients. Add about half of the buttermilk, and carefully mix to combine. We don’t want to make the dough particularly smooth. Add the remaining buttermilk, and mix, folding over with a large spoon or spatula until just combined. The dough will be thick and shaggy, but not stiff.
Transfer the dough to the parchment paper in the skillet (you can remove the paper from the skillet to shape the dough, and then return it to the skillet for baking). Using a combination of wet hands and/or a moistened silicone spatula, gently shape the dough into a disk about 10-inches in diameter. Using a sharp, wet knife, score the loaf by slicing a large “X” on the top, each cut about 2 1/2-inches deep and all the way across the loaf. Move the knife quickly through the loaf so it doesn’t drag the dough with it.
Place the skillet in the preheated oven and bake for 30 minutes. Increase the oven temperature to 400°F and bake until lightly golden brown all over and firm to the touch in the center, about another 15 minutes. Allow the loaf to rest in the skillet for about 10 minutes before lifting it out by the parchment paper and sliding it off the paper onto a wire rack to cool completely. Slice by cross-section and serve.