Brown the butter in a large pot over medium heat until it starts smelling nutty and turns a pale light brown. Then add in the marshmallows and let them melt completely, stirring, then add the Rice Krispies and sea salt; mix it all together.
Pour the mixture into a 9-by-13-inch pan and sprinkle a tiny bit more salt on top before they cool, being careful not to pack it in. If you press down on the Rice Krispies, they’ll get hard and crunchy.
Let them cool to room temp and then cut them up.