Grease a non-stick 7x11-inch baking pan well and set aside.
In a large cooking pot, melt the butter over medium heat. Once the butter is melted, add the salt and marshmallows, and stir with a rubber spatula until the mixture is smooth and melted.
Remove from the heat and immediately add the cereals. Stir together until well combined before transferring to the prepared baking pan.
Pat down gently with the back of the spatula and top with the Lucky Charms marshmallows spread evenly across the surface.
Refrigerate for 45 minutes to harden before slicing with a greased knife into 12 equal squares.
