Heat oven to 350 degrees F. Melt 3 Tbsp. butter in medium saucepan on medium heat. Whisk in flour; cook 2 min., stirring constantly. Gradually stir in milk. Bring to boil; cook and stir 3 to 5 min. or until thickened. Add VELVEETA; cook 3 min. or until melted, stirring frequently. Stir in macaroni. Spoon into 2-qt. casserole sprayed with cooking spray; sprinkle with Cheddar. Melt remaining butter; toss with cracker crumbs. Sprinkle over casserole. Bake 20 min. or until heated through.