Lightly grease a 9×13 pan and set aside.
Add butter to a saucepan. Cook on medium heat, stirring constantly, for about 5 minutes or until golden brown.
Reduce heat to low, add marshmallows to the browned butter, and cook and stir until melted.
Stir in sea salt to the mixture and remove from heat.
Stir in Rice Krispies and mix until cereal is evenly coated. Pour into the prepared baking dish. Use wax paper or greased fingers to push the cereal mix into the dish.
Sprinkle with flaked sea salt and let cool for at least 20 minutes before cutting into and serving.