Coat the bottom and sides of an 8-inch baking dish with butter. Side Note: For easier removal, you can line the dish with a bit of parchment paper if desired.
In a large saucepan, add 1 stick of unsalted butter and melt over medium heat. Stir the butter often once it has melted. It will start to foam at the top and you will see the color of the butter change to brown and start smelling nutty. This process only takes about 5 minutes and the turn happens quickly, so watch carefully.
Remove from the heat and add the marshmallows, stirring constantly until melted and smooth, about 3-4 minutes. Side Note: You may need to put the saucepan over low heat to help the marshmallows melt.
Once melted, immediately add the rice krispies and sea salt then stir until well combined.
Quickly spread the mixture in the prepared baking dish pressing it firmly into an even layer.