In a large saucepan, cook and stir butter, chocolate, Velveeta and corn syrup over medium-low heat for about 10 min. or until Velveeta is completely melted and mixture is well blended.
Mix sugar and cinnamon in large bowl. Gradually add the chocolate cheese fudge mixture, mixing well after each addition. Stir in vanilla (and nuts, if using).
Spread onto bottom of 13x9-inch pan lightly sprayed with cooking spray. Refrigerate several hours or until firm. Cut in 1-inch squares. Store in refrigerator covered with plastic wrap.