Grease a 9-inch square cake pan with softened butter. Set aside.
In a large pot, melt butter over medium-low heat. Stir until melted butter turn to medium golden brown, about 4 minutes. Stir in the marshmallows until melted and then stir in salt.
Remove pot from the heat and then gently fold in the cereal until totally combined. Quickly spread mixture into the prepared pan, pressing firmly and evenly on top and around corners and edges.