Lightly grease or butter a 9x13 inch baking pan lined with parchment paper. Set it aside.
In a large pot, melt the unsalted butter over medium heat. Stir constantly as it simmers.
As soon as brown bits start to form and the butter turns golden brown, reduce the heat and stir in the sweetened condensed milk.
Remove the pot from heat and stir in the vanilla extract and salt.
Toss in the mini marshmallows and stir continuously until they are fully melted and incorporated into the mixture.
Gently fold in the Rice Krispies cereal.
Transfer the mixture into the prepared pan and press it down with your spatula.
Let the pan sit at room temp overnight or pop the pan into the fridge for at least 1-2 hours to let everything set. Once firm, take it out, and cut into squares.
