HEAT oven to 350ºF.
MELT butter in large saucepan on low heat. Remove half of butter; mix with bread crumbs. Set aside. Stir flour and salt into remaining butter in saucepan; cook and stir 1 min. Gradually stir in milk until well blended; cook and stir until thickened. Add VELVEETA and 1-1/2 cups cheddar; cook until melted, stirring frequently. Stir in macaroni.
SPOON into 1-1/2-qt. casserole; top with remaining Cheddar and buttered crumbs.
BAKE 20 min. or until heated through.