In a large skillet (ideally with high sides), brown butter. When butter has browned, leave in skillet, but set aside.
Spread marshmallows on a baking sheet lined with parchment paper and place under the broiler. Leave the oven cracked open and keep your eyes on it! When the marshmallows just begin to smoke, and are a golden brown, remove from oven. Flip the marshmallows over, exposing an un-toasted side. Repeat until most sides are toasted. (I did this 3-4 times.)
Place marshmallows in skillet with brown butter and stir over medium-low heat to finish melting marshmallows. Transfer melted marshmallows and butter to a large mixing bowl; add cereal and toffee and stir with a wooden spoon to combine. (Do your best to evenly distribute the toffee.)
Press into a 9×13″ pan coated with cooking spray and sprinkle with sea salt. Cool before cutting and serving.