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. You can also dice the cold butter, toss it in the dry ingredients to coat in flour, then flatten each piece of butter between floured fingers.
Add the raisins or currants (or up to 1/2 cup more, as desired), and toss to coat the pieces in the dry ingredients.
Place 1 1/2 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. Sprinkle the top lightly with more flour.
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). If the dough seems to have warmed during handling, place the pan or skillet in the freezer to chill for 10 minutes or until the butter is once again firm.
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). If you've used a cast iron skillet, the bread may bake faster than if you've used a cast aluminum round baking pan.
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). Remove any paper from below the bread.
Slice and serve warm, with butter. It's also excellent the next day, toasted.
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Notes
Flour blend choicesI recommend Better Batter's original blend gluten free flour and Nicole's Best multipurpose blend with added xanthan gum. Bob's Red Mill 1-to-1 Gluten Free Baking Flour may also work, as long as you add 3/4 teaspoon additional xanthan gum since the blend has xanthan gum, but not enough. To make your own blend, visit my all purpose gluten free flour blends page for DIY “mock” recipes.