Preheat oven to 325 degrees. Line 13x9 baking pan with aluminum foil and allow end to extend over the sides of the pan.
Mix the cookie crumbs and melted butter and press firmly into the bottom of the prepared pan. You can use a measuring cup or the bottom of a glass to press the crust. Bake the crust for 10 minutes.
Beat cream cheese, sugar and vanilla in a large bowl with an electric mixer on medium speed until well blended. Add sour cream and mix well. Add eggs one at a time beating on low speed after each addition just until blended.
Remove 1 Cup of the batter and set it aside. Stir melted chocolate into remaining batter in large bowl. Pour mixture over the crust.
Top with spoonfuls of the remaining 1 Cup of plain batter.
Cut through the batters with a knife several times to create the swirl effect. Bake 40 minutes, or until center is almost set. Cool completely and refrigerate for at least 4 hours or overnight. Use the foil handles to lift the cheesecake out of the pan before cutting to serve. Keep leftovers in refrigerator.