Butter an 8-inch square baking pan. Line the pan with parchment paper so that the paper hangs over the side on two sides. Butter the bottom of the paper as well. Set aside.
Place cereal in a large bowl and set aside.
In a large, heavy bottom saucepan, melt butter over medium heat until just browned. Butter will melt, foam and begin to spit as the water cooks out of the butter. Whisk browned bits off the bottom of the pan.
Just as the butter begins to brown add the large marshmallows and the peanut butter. Stir with a spatula until the marshmallows are melted and the mixture is sticky smooth. Remove the pan from the heat and immediately pour into the cereal bowl. Add the salt and mini marshmallows and quick stir, coating all of the cereal in the peanut butter mixture.
Turn the mixture out into the prepared pan and press to fill all sides of the pan with a spatula. Let cool for at least 30 minutes before slicing into 9 or 12 squares.
Wrap individually in plastic wrap. Crispy treats will last, well wrapped, at room temperature for up to 4 days.