Preheat oven to 350 °F. Spray only the bottom of 13x9-inch pan with nonstick cooking spray.
Prepare and bake brownies according to package directions. Cool 5 to 10 minutes.
For frosting, combine confectioners' sugar, peanut butter, milk and butter in small bowl. Beat at medium speed with electric mixer until blended. Spread over warm brownies.
For drizzle, place chocolate chips and shortening in microwave-safe bowl. Microwave on high power for 1 minute or until melted. Drizzle over frosting. Allow chocolate drizzle to set before cutting into bars.