Lightly grease the bottom of a 9x13 baking pan with cooking spray and line it with parchment paper. Set aside.
Melt butter in a large pot over low heat and stir in the salt.
Add the marshmallows and stir constantly over low heat until it melts. It is ok if there are still small lumps of marshmallows.
Once the marshmallows have melted, remove the pot from the heat and add the cereal.
Fold it together with a wooden spoon until the cereal is evenly coated with the melted marshmallow mixture, being careful not to crush the cereal.
Gently spread the mixture into the prepared baking pan into an even layer. Do not press it in together too hard otherwise you will end up with hard treats.
Immediately top with extra Lucky Charms marshmallows and sprinkles for a fun touch.
Allow it to cool completely on the counter for 30 minutes to 1 hour before slicing into bars.