01 - Line a 9x13-inch baking pan with parchment paper or lightly grease with non-stick spray.
02 - In a large saucepan over low heat, melt unsalted butter, stirring occasionally.
03 - Add mini marshmallows to the melted butter and stir constantly until marshmallows are completely melted and the mixture is smooth.
04 - Remove the pan from heat. Stir in vanilla extract and a pinch of salt if desired.
05 - Immediately add crispy rice cereal. Gently fold with a spatula until cereal is evenly coated with the marshmallow mixture.
06 - Transfer the mixture to the prepared pan. Using a greased spatula or wax paper, press the mixture evenly into the pan without compacting it too firmly.
07 - Allow the mixture to cool at room temperature for at least 30 minutes before slicing into squares.