Line 13 x 9 inch pan with foil, with ends of foil extending over sides of pan. Spray with cooking spray. Prepare brownies as directed and pour into pan. Bake at 350 for 15 minutes until center is almost set.
Meanwhile, beat cream cheese, sugar and vanilla with mixer on medium speed until well-blended. Add sour cream; mix well. Add eggs, one at a time, mixing in at low speed until blended. Pour over partially baked brownie batter.
Bake 40 minutes or until center almost set. Run a knife around rim of pan to loosen cake Refrigerate 4 hours or overnight. To serve let stand at room temp 30 minutes. Lift cheesecake from pan with foil handles. Drizzle with chocolate.