Line a 9x13 baking pan with parchment paper. Lightly grease the parchment and pan with nonstick spray. Set aside.
Add the butter to a large pot over medium low heat. Let it melt completely.
Add in 6 cups of the marshmallows, and stir together gently until marshmallows are completely melted.
Remove the pan from the heat and stir in the vanilla extract and salt.
Add in the cereal, and remaining 2 cups of marshmallows and stir until the cereal is coated evenly with the melted marshmallow mixture, and the other marshmallows just start to melt.
Transfer the mixture into the prepared pan.
Using a greased rubber spatula gently press the mixture down into the pan. Do not press them hard.
Allow the treats to cool to room temperature for about 1 hour, then cut them into squares and enjoy.
Store any leftovers in an airtight container at room temperature for up to 3 days.