PREHEAT oven to 325°F. Line 13x9-inch baking pan with foil, with ends of foil extending over sides of pan; grease foil. Prepare brownie batter as directed on package; pour into prepared pan. Bake 18 to 20 min. or 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 medium speed until well blended. Add sour cream; mix well. Add eggs, one at a time, mixing on low speed after each addition just until blended. Spoon over partially baked brownie batter in pan. (Filling will come almost to top of pan.)
BAKE 40 min. or until center is almost set. Run knife or metal spatula around rim of pan to loosen cake; cool completely. 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 bars to serve. Store leftover cheesecake in refrigerator.