HEAT oven to 325°F.
PREPARE brownie batter as directed on package; pour into 13x9-inch pan sprayed with cooking spray. Bake 25 min. or until top is shiny and center is almost set.
MEANWHILE, beat cream cheese, sugar and vanilla in large bowl with mixer until well blended. Add sour cream; mix well. Add eggs, 1 at a time, mixing on low speed after each just until blended. Gently pour over brownie layer in pan. (Filling will come almost to top of pan.)
BAKE 40 min. or until center is almost set. Run knife around rim of pan to loosen sides; cool. Refrigerate 4 hours.
MELT chocolate as directed on package; drizzle over cheesecake. Refrigerate 15 min. or until chocolate is firm.