Line a 10-inch x 15-inch jelly roll pan with foil. Lightly spray the foil with non-stick cooking spray and set aside.
Melt the butter over medium-low heat in a large pot.
Reserve 4 cups of the marshmallows (see note) to the side and add the remaining marshmallows to the pot. Stir constantly until the marshmallows and butter are melted together.
Take the pot off the heat. Add in the vanilla extract and salt and stir to combine.
Add half of the cereal to the marshmallows and mix to combine. Add the remaining cereal and reserved marshmallows, and stir until well mixed.
Transfer the mixture to the prepared jelly roll pan. Lightly grease your hands with cooking spray and gently press the treats into the pan. You do not want to smash them down, just gently form them to the pan.
Let the treats set up for at least 2 hours before cutting into 24 bars and serving.
Notes
If you don't have salted butter, you can use unsalted butter and add an extra 1/2 teaspoon of salt to the recipe.
If you prefer, you can stir all of the marshmallows into the butter instead of having bits of marshmallow in the finished treats.
You can use a 9x13 baking dish instead of a jelly roll pan. The treats will be thicker and the yield will vary based on how large you cut the squares.
Store any leftovers in an airtight container for up to 5 days or freeze for up to 3 months.