Preheat oven to 350 °F. Grease bottom of 13×9-inch pan.
Prepare, bake and cool brownies according to package directions. Spread with frosting.
Place chocolate chips and shortening in microwave-safe bowl. Microwave on high power for 30 seconds, adding 15 to 30 seconds more if needed. Drizzle chocolate over frosting. Allow chocolate to set before cutting into bars.