Preheat your oven to 375°F. Grease and line a 9-inch round baking pan or line a 9-inch or 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 granulated sugar, and whisk to combine well.

Using a standard coarse grater, grate the cold butter into the dry ingredients, and mix gently to distribute the butter evenly through the mixture.

Add the raisins or currants (or up to ½ cup more, as desired), and toss to coat the pieces in the dry ingredients.

Place 1 ½ cups of the chilled buttermilk in a large measuring cup or bowl with a pour spout, add the eggs, and whisk to combine well.

Create a well in the center of the dry ingredients, add the buttermilk and egg mixture and mix gently to combine. The dough should come together.

With clean hands, knead the dough gently. If there are any spots that are dry and crumbly, add more buttermilk sparingly by the tablespoon as necessary to bring the dough together.

If using a baking pan, turn the dough out onto a lightly floured surface. If using a cast iron skillet, remove the parchment from the pan, sprinkle it lightly with flour, and turn the dough out onto that.

Pat the dough into a round that is approximately 9-inches in diameter, piling it slightly higher toward the center. Sprinkle lightly with more flour as necessary to prevent sticking.

If using a round baking pan, place the dough gently inside of it. If using a cast iron skillet, lift the paper with the dough on it and place back into the skillet.

With a very sharp knife, slice a large “X” on the top about 1-inch deep (each slash should be about 6-inches long).

Place the baking pan in the center of the preheated oven and bake until lightly golden brown all over, a toothpick inserted in the center comes out with a few moist crumbs (not wet), and the bread is firm to the touch (about 45 minutes).

Remove from the oven and allow to cool in the baking pan for 10 minutes before turning out onto a wire rack to cool until stable (30 to 60 minutes).

Slice and serve warm, with butter. It's also excellent the next day, toasted.