Prepare a 9×13-inch pan by spraying it with butter cooking spray. Set aside.
In a large pot over medium heat, melt the butter and salt. Use a rubber spatula to spread the butter up the sides to prevent the mixture from sticking.
Add the sweetened condensed milk to the melted butter and stir to combine.
Add 12 ounces of mini marshmallows. Stir over medium heat until fully melted. Then remove from the heat.
Add 1 teaspoon of vanilla and stir everything together.
Fold in the Rice Krispies cereal and the remaining 2 ounces of mini marshmallows until evenly coated.
Transfer the mixture to the prepared pan and gently press into an even layer. Avoid packing the treats too firmly.
Let cool for 1 hour before cutting into 16 squares.