Preheat your oven to 375°F. Using a large baking sheet, place parchment paper on the sheet and set aside.
In a large bowl, combine flour, xanthan gum, cornstarch, baking powder, baking soda, salt and granulated sugar.
Transfer one tablespoon of the dry ingredient mixture into a small bowl. Add the blueberries to the small bowl and toss until they are coated in the flour mixture.
Create a well in the center of the dry ingredients and add the melted and cooled butter, egg, vanilla and buttermilk. Mix until combined. Add the blueberries and gently fold them into the batter to avoid breaking their skin.
Scoop the dough into a little over two tablespoon balls, and place them about 1 1/2 inches apart from one another. Wet your hands, and press each mound down lightly to form a disk. Sprinkle each top lightly with sugar.
Place the baking sheet in the oven and bake until puffed and set in the center. There should be a light brown around the edging. Remove the baking sheet from the oven and let the cookies cool for about 10 minutes before cooling on a wire rack or paper towel.