In a large sauce pan over medium heat, melt the butter and add marshmallows, reserving 1 cup. Reduce heat to low and stir until marshmallows are melted and combined with butter into a smooth mix.
Add Reese’s puff cereal and 1 cup of mini marshmallows & gently fold cereal into melted marshmallow mixture. Be careful not to over mix and crush cereal. Transfer to a parchment paper lined baking dish and gently pat into place (avoid crushing).
Allow to cool for at least an hour before slicing and serving.