Preheat oven to 350ºF. Line an 8-inch square baking dish with parchment paper, leaving extra overhang on the sides for easy removal. Grease with butter or nonstick cooking spray.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, peanut butter, and brown sugar together until smooth, about 30 seconds. Add the egg and vanilla and continue beating. Add the flour, baking powder, and salt and stir until just combined; don't overmix.

Spread the dough evenly into the prepared pan. Bake for 18 to 20 minutes, or until the top is no longer glossy and the edges are golden brown. Remove from the oven and allow to cool completely.

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.

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!