Lightly spray a 9'' x 13'' pan with nonstick spray and set aside. Measure out the Rice Krispies and set aside.
In a large pot (a large nonstick pot, if you have one) over medium-low heat, melt the butter, chocolate chips, marshmallows, and peanut butter. Stir periodically, making sure the mixture isn’t scorching on the bottom of the pan, until just combined.
Remove pot from heat and add the Rice Krispies, quickly folding to evenly combined.
Spread mixture into prepared pan, gently pressing to create an even layer. I like to save the butter wrapper and use the buttery side when pressing = no sticking! Take care to not press too hard, to compact the bars, as this can make them too firm. Let cool at room temperature and then cut into squares.
