3/4 cup (120 g) superfine white rice flour
1/4 cup (36 g) cornstarch (or 1 full cup superfine white rice flour)
1/4 teaspoon xanthan gum (optional)
1/2 teaspoon baking powder
1/4 teaspoon baking soda
1/8 teaspoon kosher salt
1/4 cup (50 g) granulated sugar
2 tablespoons (28 g) packed light brown sugar
1/2 cup (4 fluid ounces) buttermilk, at room temperature
2 eggs (100 g, weighed out of shell) at room temperature, beaten
4 tablespoons (56 g) unsalted butter, at room temperature
4 ounces blueberries (fresh or frozen)
Coarse sugar, for sprinkling (optional)
Preheat the oven to 350°F. Grease or line 6 standard-size wells of a muffin tin, and set the tin aside.
In a large bowl, place the rice flour (or rice flour and cornstarch), (optional) xanthan gum, baking powder, baking soda, salt, and granulated sugar, and whisk to combine well. Add the brown sugar, and mix to combine, breaking up any lumps. Create a well in the center of the dry ingredients and add the buttermilk, eggs, and butter, and mix to combine. The batter should be smooth and glossy (and rather yellow). Add most of the blueberries, reserving a few for placing on top of the batter in the wells.
Divide the batter among the prepared wells of the muffin tin, filling each well almost completely full. Add any reserved berries to any tops that look rather bare. Sprinkle the tops very lightly with the (optional) coarse sugar.
Place the tin in the center of the preheated oven and bake until domed, pale golden, and the top of each muffin springs back when pressed gently in the center (about 20 minutes). Allow the muffins to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.