1. Preheat oven to 325 degrees. Line 13x9 pan with foil, with ends of foil extending over sides of pan. Mix cookie crumbs and butter. Press firmly into bottom of prepared pan. Bake 10 minutes.
2. Beat cream cheese, sugar and vanilla in large bowl with electric mixer on medium speed until well blended. Add sour cream; mix well. Add eggs, one at a time, beating on low speed after each addition just until blended. Remove one cup of the batter; set aside. Stir melted chocolate into remaining batter in large bowl. Pour over crust. Top with spoonfuls of the remaining cup of plain batter; cut through batters with knife several times for swirling effect.
3. Bake 40 minutes or until center is almost set. Cool. Refrigerate at least four hours or overnight. Use foil handles to lift cheesecake from pan before cutting to serve.
