In a medium microwave-safe bowl, heat the peanut butter and white chocolate in the microwave for 1 minute, remove, stir, then return to the microwave for another 30 seconds. Stir until smooth, then add the cereal and chopped nuts, stirring until incorporated. Spread mixture evenly on top of the cooled blondies. Refrigerate while you work on the rice krispy layer.
Make the Rice Krispies Treats:
In a medium saucepan, heat the butter until fully melted. Add 3 cups of the mini marshmallows, stirring until marshmallows are mostly melted. Pour in the cereal, stirring until combined. Turn off the heat and stir in the remaining 1/2 cup of marshmallows. Dump mixture on top of the peanut butter krispies layer, using a spatula to gently pack it into an even layer. Refrigerate for 1 hour to allow all layers to set up.
Run a knife around the sides of the pan to loosen. Using parchment overhang, lift the treats from the pan and onto a cutting board. Slice into squares, serve, and enjoy!
*This recipe can easily be doubled and made in a 9×13-inch baking dish.