Preheat your oven to 375°F. Grease a baking dish with an 8-cup to 10-cup capacity lightly and set it aside. I used an oval casserole pan, but a 9-inch square or round baking pan works fine.
Make the pastry dough.
In a large bowl, place the gluten free flour blend, xanthan gum, baking powder, baking soda, salt and 2 tablespoons sugar, and whisk to combine well.
Add the grated butter, and toss to combine the butter in the dry ingredients.
Create a well in the center of the dry ingredients, add the buttermilk and mix until just combined. The mixture will be thick but much softer than biscuits.
Place bowl with the topping in the refrigerator to chill briefly, while you make the filling.
Prepare the filling
In a large bowl, place the blueberries, 1/2 cup sugar, 1/4 cup rice flour (or alternative), salt, and (optional) cinnamon, vanilla extract and 2 tablespoons of melted butter, and toss to combine completely.
Mix in water by the drop as necessary just to moisten all of the flour and sugar.
Transfer the blueberry mixture to the prepared baking dish and spread it into an even layer.
Finish the cobbler.
Remove the topping from the refrigerator. Using two spoons or a spring loaded ice cream scoop, scatter the topping evenly over the top of the blueberries.
Using a spatula or knife, spread the topping out into a craggy somewhat even layer, leaving a few small gaps where the blueberries poke through.
Brush the pastry topping evenly with the 1 tablespoon of melted butter.
Sprinkle the remaining 2 tablespoons of sugar on top of the melted butter.
Place the baking dish in the center of the preheated oven and bake for about 35 minutes or until done.
The cobbler is done when the filling is bubbling, the top is golden brown all over and a toothpick inserted just into the topping comes out clean.
Remove the baking dish from the oven and allow to cool for 10 to 15 minutes before serving, so the filling and topping can set up.
Notes
Best flour blends for the topping Use a high-quality all purpose gluten free flour blend like Better Batter's original blend or Nicole’s Best with added xanthan gum. Bob’s Red Mill 1-to-1 works if you add ¼ teaspoon xanthan gum—otherwise the topping may be crumbly. I don’t recommend Cup4Cup anymore due to a recent formula change.Want to make your own blend? See my all purpose gluten free flour blends page.What to use in the filling Stick to single-ingredient flours like superfine white rice flour, tapioca starch, or cornstarch. Avoid blends with xanthan gum—they’ll make the filling gummy.
Substitutions
For dairy free, use vegan butter and 1 cup nondairy buttermilk made by whisking 1/2 cup unflavored nondairy milk with 1/2 cup nondairy plain yogurt.