Add the butter to a large stock pot over medium heat. Stir frequently. Remove from heat once you see brown flecks.
Add vanilla, sea salt and the marshmallows. Place the pot back on the burner and stir until marshmallows are melted.
Once marshmallows are melted, remove the pot from the heat and add the cereal. Stir until fully combined.
Scrape the mixture in the greased baking dish and press the mixture down firmly with greased fingers. Place in the fridge for at least 30 minutes before serving. Sprinkle with more sea salt once set.