Spray a 13 X 9-inch pan with no-stick cooking spray.
In a medium bowl combine 1 cup of peanut butter, water and egg in large bowl, stirring until smooth. Stir in cake mix until blended well. Spread into prepared pan.
BAKE 20 to 25 minutes or until puffed and light golden brown. Cool completely.
Mix all ingredients with a hand mixer until smooth. Add more powdered sugar or milk until you reach desired consistency and spread on top of bars.
Notes
You can make these peanut butter treats even more delicious by sprinkling either some peanut butter chips or white chocolate chips on top.