PREHEAT oven to 350°F. Line 13x9-inch baking pan with foil, with ends of foil extending over sides of pan; grease foil. Microwave unsweetened chocolate and butter in large microwaveable bowl on HIGH 2 min. or until butter is melted. Stir until chocolate is completely melted. Add granulated sugar; mix well. Blend in eggs and 1 tsp. of the vanilla. Stir in flour until well blended. Spread into prepared pan.
BAKE 30 to 35 min. or until wooden toothpick inserted in center comes out with fudgy crumbs. (Do not overbake.) Cool in pan on wire rack.
BEAT peanut butter, powdered sugar and remaining 1 tsp. vanilla until well blended. Spread over brownie. Microwave semi-sweet chocolate in microwaveable bowl on HIGH 2 min. Stir until chocolate is completely melted. Drizzle chocolate from spoon over peanut butter layer. Refrigerate 30 min. or until chocolate is firm. Remove brownie from pan, using foil handles. Cut into 36 brownies to serve.