Pour cereal and 2 cups of marshmallows into an extra-large bowl. Grease a 13x18’ half sheet pan lightly with butter or non-stick cooking spray. Set aside.

Melt (or brown) the butter: Add butter to a large saucepan and allow to melt. (optional: brown the butter by cooking it over low heat, swirling it around a few times, until it’s golden and smells nutty fragrant--don’t let it burn!).

Remove from heat and stir in remaining marshmallows, vanilla and salt. Stir constantly, and place back on low heat if needed, until melted and smooth.

Gradually pour mixture over the cereal, tossing to coat evenly. Stir in other mix-ins* if desired.

Press mixture firmly into prepared pan and allow to cool before cutting into squares. (I cut 20 squares (5 by 4).