HEAT oven to 350°F.
MELT butter in large saucepan on low heat. Stir in flour; cook and stir 2 min. or until bubbly. Gradually stir in milk; bring to boil on medium heat, stirring constantly. Simmer on low heat 3 to 5 min. or until thickened, stirring constantly. Add 1-1/2 cups cheese; cook and stir 2 to 3 min. or until melted. Stir in macaroni.
SPOON into 1-1/2-qt. casserole sprayed with cooking spray; sprinkle with remaining cheese.
BAKE 20 min. or until heated through.