For Crust: Preheat oven to 350 degrees. Place butter and peanut butter in small bowl and heat uncovered on high for 1 minute in microwave or until butter is melted. Stir well until blended. Place cake mix in medium bowl and whisk to break up lumps. Add peanut butter mixture and stir until a thick dough forms.
Break apart dough in to sections and distribute over bottom of ungreased large sheet pan. Pat dough into pan: spread evenly using your hands. Bake 15-17 minutes or until crust is set and golden brown. Remove from oven and let cool for 2 minutes.
Meanwhile, combine peanut butter, corn syrup and butter in 3 qt.pan. Cook over medium heat, stirring constantly until mixture is smooth. Spoon 3/4 cup of the peanut mixture over hot crust and spread evenly.Spread Marshmallows evenly over the top. Return to oven and bake and additional 1- 2 minutes until marshmallows begin to puff a little. Remove pan from oven.
Add cereal and peanuts to the remaining peanut butter mixture and spoon over Marshmallows and spread evenly. Cool completely or until set and cut into bars.