Sift the rice flour and tapioca flour into a large bowl. Add the salt. Make a well in the center and drop in the eggs. Using a small whisk or wooden spoon, stir continuously, gradually drawing in flour from the sides, adding half the milk in a steady stream at the same time. When all the flour has been mixed in, whisk in the remainder of the milk and the cool melted butter. Allow to stand for 1 hour.