HEAT oven to 375ºF.
MICROWAVE unsweetened chocolate and oil in large microwaveable bowl on HIGH 45 sec. to 1 min. or until chocolate is almost melted; stir with whisk until chocolate is completely melted.
ADD next 6 ingredients; mix well. Stir in semi-sweet chocolate. Spread onto bottom of 9-inch square pan spayed with cooking spray.
BAKE 30 to 35 min. or until toothpick inserted in center comes out clean. Run knife around sides of pan to loosen cake; cool completely.
MIX peanut butter and powdered sugar until blended; spread onto cake.