Cook the macaroni in boiling salted water until tender, drain and rinse.
Melt the butter and add the flour. Cook for a minute or two over low heat and then add warmed milk and whisk till smooth. Let thicken over heat and then add 2/3 of the cheese and stir until melted.
Pour the entire combination over the cooked macaroni in a greased baking dish and sprinkle with remaining cheese over the top and bake for 30 minutes.