Use non-stick cooking spray or butter to grease a 9in (23 x 23cm) square cake pan with 2in (5cm) sides.
Brown your butter ina large pot over medium-low heat. Stir and scrape the bottom of the pan frequently until your butter turns golden and the milk solids separate and turn brown. You will be able to smell the nuttiness in the brown butter. Be careful brown butter is easily burned.
Turn in the heat down to the lowest setting and quickly add your marshmallows. Using a wooden spoon melt your marshmallows stirring continuously until the marshmallows are smooth and melted.
Remove from heat and stir in your salt and rice cereal together quickly. Once incorporated immediately spread in your prepared pan. Be careful the mixture is very hot. We like to wait a minute then using lightly greased hands or a greased piece of parchment press it down into the pan.
Let it cool. Then cut into even squares and serve with an extra sprinkle of sea salt