Line a 9x13-inch baking pan with wax paper or spray it with cooking spray. Set aside 1 cup of the marshmallows.
Brown the butter. Melt butter in a pan over medium heat. Stir often as it melts.
When the butter foams up and turns bubbly, stir quickly until brown bits begin forming at the bottom of the pan. Keep stirring. Shake the pan if necessary to keep the liquid moving.
Remove from heat when butter begins to smell like hazelnuts and the liquid is light brown in color, about 10 minutes.
Stir in the marshmallows (minus 1 cup), bourbon, vanilla and salt until combined.
Lastly, stir in the cereal. Once it's mixed in, stir in the remaining 1 cup of marshmallows.
Press in the rice krispie treat mixture. Use the back of a spoon to press it in compactly. Sprinkle with flakes of fresh sea salt.