Preheat oven to 350°F. Line 13x9" baking pan with foil, with ends of foil extending over sides of pan. Spray with cooking spray. Prepare brownie batter as directed on package, pour into prepared pan. Bake 15 min. of until top of brownie is shiny and center is almost set.
Meanwhile, beat cream cheese, sugar, and vanilla in large bowl with electric mixer on med. speed until well blended. Add sour cream, mix well. Add eggs, 1 at a time, mixing on low speed after each additional just until blended. Pour over partially baked brownie batter in pan.
Bake 40 min. or until center is almost set. Cool. Refrigerate 4 hours or overnight. Let stand at room temperature 30 min. before serving. Lift cheesecake from pan, using foil handles. Drizzle with chocolate, let stand until chocolate is firm. Cut into 16 pieces to serve. Store leftover Brownie Cheesecake in refrigerator.