In a saucepan, melt butter over low heat. Stir in flour until smooth. Gradually add milk; cook and stir over medium heat until mixture thickens. Remove from the heat. Add the salt, cheddar cheese and Parmesan cheese, stir until cheeses melt. Add potatoes; stir gently to mix. Place in a greased 2-qt. Baking dish. Sprinkle bread crumbs on top. Bake, uncovered, at 350! for 30-35 minutes.