Grease a 9x13-inch baking pan with butter or non-stick spray and set it aside.
Melt the marshmallows and butter together — either on the stovetop over low heat, stirring gently and constantly until smooth, or in the microwave in 15-second bursts, stirring between each round.
Once smooth, stir in the salt and vanilla if using.
Fold in the cereal, mixing gently until every piece is evenly coated in the marshmallow mixture.
Transfer to the prepared pan, press down lightly and evenly with a spatula, then let cool for about 15 minutes before cutting into bars or shapes and serving.
Notes
Melt marshmallows and butter over low heat or in short microwave bursts to keep treats soft.
Fold in the cereal gently rather than stirring hard, to avoid crushing it.
Press the mixture into the pan lightly — pressing too hard makes treats dense and tough.
Cut while just barely warm, not fully cooled, for the cleanest slices.
Store at room temperature, not the fridge, to keep treats soft for several days.
Grease your hands or spatula to keep the sticky mixture from sticking while pressing.