Line a 9 inch square pan with nonstick foil. Alternatively, line with regular foil and spray with cooking spray.

In a large (3 quart), preferably non-stick saucepan or a slightly larger pot, melt the butter over medium heat. When it's about halfway melted, add the salt and swirl it around.

Add the marshmallows and stir over medium heat until they're about halfway melted, then reduce heat slightly and continue stirring until they are fully melted.

Remove from heat and stir in the vanilla extract.

Add the cereal (it will seem like a lot) and stir until fully mixed, then press into the pan and let cool completely.

Lift from pan and cut into squares.