Step 1

Heat oven to 350°F.

Step 2

Line 13x9-inch pan with foil, with ends of foil extending over sides. Spray with cooking spray. Microwave unsweetened chocolate and butter in microwaveable bowl on HIGH 2 min. or until butter is melted. Stir until chocolate is completely melted and mixture is well blended. Add 2 cups sugar; mix well. Stir in 3 eggs and 1 tsp. vanilla. Add flour; mix well. Spread onto bottom of prepared pan.

Step 3

Bake 20 min. Meanwhile, beat cream cheese, and remaining sugar and vanilla in large bowl with mixer until blended. Add sour cream; mix well. Add remaining eggs, 1 at a time, mixing on low speed after each just until blended.

Step 4

Pour cream cheese batter over partially baked brownie. (Cream cheese mixture will come almost to top of pan.) Bake 40 min. or until center is almost set. Run knife around rim of pan to loosen cake; cool.

Step 5

Refrigerate cheesecake 4 hours. Let stand at room temperature 30 min. before serving. Meanwhile, melt semi-sweet chocolate as directed on package.

Step 6

Use foil handles to lift cheesecake from pan. Drizzle with chocolate; let stand until firm before cutting cheesecake into bars.