Line 13x9-inch pan with foil, with ends of foil extending over sides. Mix graham crumbs, butter and 2 Tbsp. sugar; press onto bottom of pan. Refrigerate while preparing filling.
Melt 6 oz. chocolate as directed on package; cool slightly. Beat cream cheese, 1/2 cup sugar and vanilla in large bowl with mixer until blended. Gently stir in COOL WHIP. Pour half the batter into medium bowl; stir in melted chocolate. Pour over crust; cover with remaining plain batter. Melt remaining chocolate; drizzle over batter.
Refrigerate 3 hours or until firm. Use foil handles to remove cheesecake from pan before cutting to serve.