Butter a 9″ x 13″ baking dish and set aside.

In a large pot, warm butter over medium low heat.  The butter will melt, then foam and bubble, then start to brown.  You will notice little brown bits forming at the bottom of the pan.  Stir frequently during this process and keep your attention on the pot.  The butter can go from wonderfully brown to burnt in just a few moments.  You will know the butter is done when it gives off a nutty aroma and turns a golden brown (check out this great resource for how to brown butter).

Turn off the heat and add the marshmallows, stirring until smooth.  Add half of the cereal and stir to incorporate.  Sprinkle in the flaky sea salt and remaining cereal and continue stirring until everything is fully mixed together.

Transfer mixture to the prepared baking dish.  Using a silicon spatula (or piece of parchment paper), press mixture into the dish in an even layer.  Sprinkle with additional flaky sea salt to taste.