HEAT oven to 350°F. Coat bottom of 13 x 9-inch baking pan with no-stick cooking spray.
PREPARE brownie mix batter according to package instructions using oil, water and eggs. Spread evenly in prepared pan.
MICROWAVE peanut butter in medium microwave-safe bowl on HIGH 30 seconds. Add powdered sugar. Whisk until smooth. Drop by teaspoonfuls onto brownie batter and gently swirl with tip of knife.
BAKE 29 to 32 minutes or until set in center. Cool completely in pan on wire rack.