01 - Line a 9x13-inch baking pan with parchment paper or lightly grease it.
02 - In a large saucepan over low heat, melt the unsalted butter.
03 - Add mini marshmallows to the melted butter and stir continuously until smooth and fully melted.
04 - Remove from heat, then stir in vanilla extract and salt if desired.
05 - Fold in crispy rice cereal gently until evenly coated with the marshmallow mixture.
06 - Quickly transfer mixture to the prepared pan and press evenly using a buttered spatula or wax paper.
07 - If using, sprinkle mini chocolate chips over the top and press gently.
08 - Allow bars to cool at room temperature for approximately 30 minutes until set.
09 - Cut into 16 bars and serve as desired.