Velveeta Down-Home Macaroni & Cheese

1/4 cup (1/2 stick) butter or margarine, divided
1/4 cup flour
1 cup milk
1/2 lb. (8 oz.) VELVEETA Pasteurized Prepared Cheese Product, cut into 1/2-inch cubes
2 cups elbow macaroni, cooked, drained
1/2 cup KRAFT Shredded Cheddar Cheese
6 RITZ Crackers, crushed

HEAT oven to 350°F. Melt 3 Tbsp. butter in medium saucepan on medium heat. Whisk in flour; cook 2 min., stirring constantly. Gradually stir in milk; cook until mixture boils and thickens, stirring constantly. 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.