In a small saucepan, heat the butter over medium high heat. Once the butter is completely melted, continue to whisk it until it starts to bubble and then starts to turn golden brown. Keep your eyes in it, as it's super easy to burn it when you're making brown butter.
Once the butter turns a golden brown color, and smells nutty, reduce the heat to medium and add the marshmallows to the brown butter. Stir the marshmallows until completely melted and then add the rice krispie cereal. Using a wooden spoon, stir everything together until evenly combined.
Spray a large baking dish with non-stick spray and transfer the rice krispie treats mixture to the dish. I like to spray my spatula with a bit of non-stick spray and then use it to evenly press down the treats into a uniform layer.
Spread the entire jar of peanut butter on top of the brown butter rice krispies and then transfer it into the refrigerator for at least 30 minutes to chill.
Meanwhile, heat the heavy cream in a small saucepan until just barely starting to boil. Remove from heat and add the chocolate chips. Let the chocolate chips melt for a few moments, and then whisk the mixture together until smooth.
Remove the baking sheet from the refrigerator and pour on the ganache. Spread the ganache on top and then place the entire dish back into the fridge for at least another hour.
Dust with flakey salt before cutting and serving.