Butter and line a 9x9 baking dish with parchment paper (you can use a 9x13 but the squares won’t be as thick).
In a large pot or dutch oven, melt butter over medium/low heat. Add 6 cups marshmallows and stir just until completely melted (don’t overcook or the mixture will harden).
Remove the pan from the heat and immediately stir in vanilla and salt. Working quickly add the cereal and remaining 2 cups marshmallows and stir/ fold together until well coated and added marshmallows just start to melt but are still chunky.
Immediately transfer the mixture to the prepared pan, place a piece of plastic wrap or parchment paper over the top to prevent sticking and lightly pressing it evenly into the pan.
Set aside to cool completely to room temperature (about 1 hour), then slice into squares and enjoy.
