Melt butter in a pan over low heat.
Stir in marshmallows, reserving 2 cups to add later.
Stir regularly until marshmallows have melted. Turn off heat and stir in rice krispy cereal and sprinkle with sea salt.
Add remaining 2 cups of marshmallows and stir. These won't melt completely with the rest, but will make your treats extra gooey!
Add to a greased pan and allow to cool before cutting.