In a medium mixing bowl, whisk together white rice flour, cornstarch, sweet rice flour, granulated sugar, baking powder, salt, and nutmeg.
Add the milk, vegetable oil, egg, and vanilla extract. Whisk to combine. The batter will be very, very, very thin. I just use a large fork to whisk it together. There really is no need to dirty your electric mixer for this recipe. Fill muffin cups about 1/2 full.
While muffins are baking, fill a 8x8x2-inch cake pan with powdered sugar. Remove muffins from the oven and place directly into the powdered sugar. The steam from the hot muffins will make the sugar stick to the muffins.
Cover the muffins in powdered sugar and allow to cool. Remove muffins from the sugar and tap off any excess sugar.