Coat a 10x8 or 9×13 pan with butter. Place a piece of parchment paper across the bottom and up the side to create a sling. This will make it easier to remove the treats once they are set.
Add the peanut butter and butter to a large pot over medium-high heat, constantly stirring while they melt together.
Once the peanut butter and butter are fully melted and smooth, turn down the heat to medium, add the marshmallows, and continue to stir. The marshmallows will fully melt into the mixture, this takes a few minutes.
Remove the pot from the heat and pour in the rice cereal. Quickly stir everything together to coat all of the cereal. You must work quickly because the mixture will start to harden as it cools.
Pour the mixture into the prepared pan. Once cool enough to touch, gently press down on the top of the mixture to compact the treats. You can also use the bottom of a cup.
While the treats are cooling, place the chocolate into a microwave-safe bowl. Microwave for 60 seconds, remove from the microwave and give it a stir. Continue to melt in 30-second intervals, stirring in between, until the chocolate is fully melted, this should only take another 60-90 seconds.
Carefully pour the melted chocolate on top of the treats and spread it into an even layer with the back of a spoon. Sprinkle the top with salt and allow the bars to cool for a minimum of 1 hour, 2 is best to allow the chocolate to set.
When ready to serve, remove the bars from the pan and cut them into squares. Sprinkle the tops with a little more salt if desired and enjoy!