Spray a 9 x 13 inch pan with cooking spray. Set aside.
Add the butter to a Dutch oven or large saucepan over medium heat. Melt butter until bubbly. Allow the butter to keep bubbling until the bubbles turn into a fine foam. At this point, whisk the mixture continuously and keep a close eye on it. It’ll go fast from here! When you see brown bits forming on the bottom of the pot, remove the butter from the heat. Keep whisking for 30 seconds.
Return the pot to low heat, and stir in the sweetened condensed milk, vanilla, and pinch salt.
Add 7 cups of the marshmallows and cook, stirring often, until they are completely melted.
Remove from heat and add the cereal, gently stirring the mixture together. Sprinkle in the final cup of marshmallows and keep stirring until all the cereal is coated and extra marshmallows are just starting to melt.
Pour the cereal mixture into the pan. Spray your hands with cooking spray and gently pat the mixture evenly into the pan. Allow to cool at least 15 minutes before slicing.
Taste, sprinkle with additional flaky salt if desired, and serve.