Grease a 9×13″ baking pan with margarine or oil.

In a large saucepan over low heat, melt the butter. Add the marshmallows and continue to cook over low heat, stirring until the marshmallows are melted. Add the vanilla and stir.

Add the cereal and stir to combine.

Using a rubber spatula, spread the mixture in the prepared pan and pat down with your hands to flatten (apply some oil or water to your hands first to prevent it from sticking).

Set aside to cool before digging in!