MICROWAVE VELVEETA, butter, chocolate and corn syrup in microwaveable bowl on HIGH 3 min. or until blended, stirring after 2 min.
ADD chocolate mixture, in batches, to sugar in large bowl, beating with mixer on medium speed until blended after each addition. Stir in nuts and vanilla.
POUR into 13x9-inch pan sprayed with cooking spray. Smooth top with spatula. Refrigerate several hours or until firm before cutting into 1-inch squares.