Grease an 8x8-inch baking pan with cooking spray and line it with parchment paper, leaving a 1-inch overhang on each side. Set aside.
Heat the butter in a small stockpot or very large saucepan over low heat 1 minute or until melted.
Increase heat to medium and cook, stirring constantly, 7 to 10 minutes or until you notice the butter turning a dark golden brown color along the bottom of the pan.
Immediately add marshmallows and salt, stirring constantly, until fully melted into the butter. Once the marshmallows are completely melted, remove from the heat and stir in the cereal.
Gently fold the cereal and marshmallow mixture together until the cereal is fully combined. Fold in the peanut butter, being careful to not crush the cereal.
Scoop the cereal into the prepared pan, pressing down as you work with your hand. Be sure to get the cereal into each corner.
If desired, spread additional peanut butter on top of the rice krispies. Let the rice krispie treats sit at room temperature for at least 1 hour or until set.
Use the parchment paper overhang to slide the bars out of the baking pan, slice, and serve.