Put the butter in a pot and bring it to a steady boil. Add the rice flour all at once, stirring until it's no longer floury and is heated through.
While it's simmering, add one ladle of cold milk and mix. When the roux comes together, add another ladle of milk and mix it in. Repeat these steps.
When you've added all of the milk, add the salt and stir well from the bottom of the pot. When it thickens and comes to a simmer, it's done! If you have fresh cream, add it at the end of cooking and mix it in.
Mix this sauce with any ingredients you like to make gratin. You could also use it for making doria or croquettes.
Story Behind this Recipe
I'm in the process of eliminating wheat for my family.
If you're used to making sauces, you can use medium heat. If you're worried about it burning, I think it would be better cook on low.