Step 1

Heat oven to 350°F. Grease bottom of 13x9-inch pan with shortening or cooking spray. Make brownie batter as directed on box. Spread in pan.

Step 2

In small microwavable bowl, microwave peanut butter uncovered on High 25 seconds; stir until smooth. Drop 20 spoonfuls of peanut butter in 5x4 rows over brownie batter. For marbled design, pull knife through batter and filling about 1 inch apart; turn pan and repeat.

Step 3

Bake 19 to 22 minutes or until toothpick inserted 2 inches from side of pan comes out clean. Cool completely on cooling rack, about 2 hours. Store covered at room temperature.