Brown Butter–Bourbon Rice Krispies Treats

Grease a 9×13-inch baking pan with butter. Set aside.
In a very large pot set over medium-high heat, melt butter, stirring constantly, until the milk solids at the bottom of the pan start to brown and the butter smells nutty and darkens in color. This will take about 5 minutes. Do not walk away or leave unattended.
As soon as the butter has browned, add two bags of marshmallows, the bourbon, vanilla, and salt. Cook for 5 to 6 minutes, stirring constantly, until the mixture is golden, smells like caramel, and is smooth and glossy.
Turn off the heat, add the remaining bag of marshmallows, and stir just until melted. Add the rice cereal and quickly stir to coat them with the marshmallow mixture. (If your pot is not big enough, quickly transfer the mixture into the largest mixing bowl you have.)
Transfer into the prepared baking pan and gently press into an even layer. If the mixture is too sticky, coat your hands in butter or oil to make it easier to work with. Garnish with flaky salt. Let cool completely and cut into squares.