Heat oven to 350 degrees F. Lightly spray 13 x 9-inch pan with no-stick cooking spray. In large bowl, combine cake mix, 1 cup peanut butter, water and egg on low speed, mixing 2 minutes on medium speed.
Spread into prepared pan.
Bake at 350 degrees F for 20 to 25 minutes or until puffed and light golden brown. Cool completely.
Blend frosting with 1/2 cup peanut butter in a small bowl.