Lightly spray a 9×13-inch pan with nonstick spray and set aside.
Place the butter in a large pot over medium heat. Cook, stirring frequently, until the butter foams and the milk solids turn golden brown and smell nutty, about 4 to 6 minutes. Remove from the heat.
Add 14 ounces of the mini marshmallows to the browned butter and stir until melted and smooth.
Stir in the vanilla and kosher salt.
Add the Rice Krispies cereal and mix until evenly coated.
Fold in the remaining 2 ounces of mini marshmallows.
Transfer the mixture to the prepared pan and lightly press into an even layer. Do not pack the mixture tightly.
Allow the treats to cool for 1 hour before cutting into 16 squares.