Preheat your oven to 350°F. Grease and line a standard 9 x 5-inch loaf pan and set it aside.
In the bowl of a stand mixer fitted with the paddle attachment, or a large bowl with a handheld mixer, place the butter and 1 cup of sugar. Beat on medium-high speed until light and fluffy.
Add the eggs and beat on medium speed until smooth.
In a separate, medium-size bowl, place the flour blend, xanthan gum, baking powder, baking soda and salt, and whisk to combine well.
Place the cranberry halves in a separate, small bowl. Add about one tablespoon of the dry ingredients to the cranberries, and toss to coat. Set the cranberries aside.
To the bowl with the butter and sugar mixture, add the dry ingredients in two parts, alternating with the buttermilk, beating after each addition. Be sure to begin and end with the dry ingredients. The mixture will be thick but smooth.
Add the cranberries and reserved dry ingredients, and mix gently into the batter until evenly distributed throughout.
Scrape the batter into the prepared pan, and smooth the top. It will most or completely fill the pan, depending on the exact depth of your loaf pan.
Smooth the top with a wet spatula, score down the center of the raw loaf about 1/4-inch deep with a knife or offset spatula. Sprinkle the top with the remaining tablespoon of sugar.
Place the pan in the center of the preheated oven and bake, rotating once, until golden brown on top and a toothpick inserted in the center comes out clean (about 1 hour).
Remove from the oven and allow to cool in the pan for 20 minutes before transferring to a wire rack to cool completely.
Slice and serve.
Notes
About the buttermilk.The buttermilk can be replaced by 1/2 cup (4 fluid ounces) milk and 1/2 cup sour cream (120 g) or 1 cup (227 g) plain whole milk yogurt. Nutrition information is an estimate based on a full loaf divided into 10 equal slices.