PLACE VELVEETA, butter, chocolate and corn syrup in large microwaveable bowl. Microwave on HIGH 2 min.; stir. Microwave an additional minute; stir until well blended.
ADD chocolate mixture, in batches, to sugar in large bowl, beating with electric mixer on medium speed until well blended after each addition. Stir in pecans and vanilla.
POUR into greased 13x9-inch pan. Smooth top with spatula; cover. Refrigerate several hours or until firm before cutting into 1-inch squares to serve. (For longer storage, wrap tightly and freeze up to 2 months. Thaw in refrigerator overnight before serving.)